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Pelican Technical Article:

About Oxygen Sensors
Jared Fenton
Wayne R. Dempsey

Difficulty Level: 1
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten

     In this article, we will discuss oxygen sensors, what they do and how to test oxygen sensors. I will try to explain a little about them and answer a few questions. Keep in mind that this article is not vehicle specific, but applies to all cars in general. As always, if you are in doubt about something, consult your owner’s manual or repair guide for vehicle specific information.

When should I replace the oxygen sensor?

     Usually, oxygen sensors require replacement around 60 to 100K miles. You should check the owner’s manual or repair guide for the recommended mileage for your car. Most modern cars have a service light or gauge that lights up when the preset mileage has been reached. This light is usually triggered by a few different things, such as a mechanical mileage counter or by counting a certain amount of turns of the key in the ignition. In these cases, the light is merely a reminder to service the sensor. Be sure not to confuse a service reminder light with an actually failure light. A failure light is a light that only illuminates when the sensor actually fails. Check the owner’s manual or repair guide if you are in any doubt as to whether or not the light is a failure light or service light.  It is not always necessary to replace the sensor at the factory recommended mileage however. These are usually just a service recommendation. Naturally, parts do wear out and fail over time, so it’s always a good idea to practice preventative maintenance on your car.

What are the symptoms of a bad oxygen sensor?

     The biggest indicator of a faulty oxygen sensor is a noticeable decrease in fuel economy, along with a rich mixture. Now, this does not automatically indicate that the sensor has failed. Be sure to check all vacuum hoses for leaks as well as the ignition system, check the plugs, (are they fouled?) check the rotor, distributor cap, points, spark plug leads, and condenser (check all of these where applicable) Vacuum leaks and ignition problems are notorious for causing fuel economy problems. It’s a good idea to inspect and re-new the vacuum hoses every couple of years anyway. Other symptoms of a faulty sensor are a loss of power, (particularly when accelerating from a stand-still), overheating, and spark plug fouling, both of these are due to an increased rich or lean running condition. 

     Most modern cars nowadays come with sort of a built in troubleshooting guide that can help you diagnose if the sensor is bad. There is usually a port or connector where you can plug in a computer and extract codes from the fuel injection computer. In the case of most new cars, when an error occurs, it generates a code and stores it. Using the computer, you can extract the code from the fuel injection computer. This is invaluable in determining not only a bad oxygen sensor, but all sorts of other problems as well. In many cases, this can tell you if the sensor is bad, however if it does not, keep reading, the steps below will go over how to test the sensor.

What can damage the oxygen sensor?

     Sometimes, but not always, certain home or shop repairs can possibly damage an oxygen sensor. Usually, the sensor is placed in the exhaust manifold very near the bottom of the engine. It is in this area that you have components such as oil pans, valve covers, and other panels and covers that are usually sealed with a silicone-based sealer such as RTV. Some of these sealers contain fumes that will damage the sensor. If you are working in the area of the sensor, be sure to use a silicone-based sealer that is labeled “Oxygen Sensor Safe” You can find this sealer at any auto parts store. Other things that can destroy a sensor are anti-freeze/coolant, using leaded fuel (even though it is hard to find these days), as well as a sustained rich running engine. (This is usually the case when the oxygen sensor fails) When the engine is running rich for an extended amount of time, carbon will begin to build up on the inlets for the sensor, clog it, and eventually burn it out.

Is it safe to test an oxygen sensor?

     As long as you are merely testing the voltage output, it is usually safe to test the sensor. You do not want to apply any sort of voltage to the sensor, this can fry the sensor. Also, you do not want to check resistance between terminals as checking resistance means that you are sending voltage into a circuit, and measuring the amount returning. This can fry the sensor as well.

How does this thing work?

     Oxygen sensors are essentially chemical generators. They work by constantly measuring the oxygen content inside the exhaust manifold and comparing it to the air outside the engine. If this comparison shows little or no oxygen in the exhaust manifold, a voltage is generated. This voltage is then sent to the fuel injection computer, where it is received and based on the voltage, the fuel injection computer makes the necessary adjustments to change the overall fuel-air mixture of the engine. When the oxygen sensor measures the correct mixture, the voltage drops and it sends the appropriate signal to the computer to stop adjusting the mixture. This is a non-stop exchange of signals between the sensor and the computer, and it is constantly making adjustments depending on the needs of the engine. When the sensor fails, it stops sending voltage to the fuel injection computer, and usually the computer interprets this as “ok, we aren’t getting a signal, so we better enrich this engine as much as we can”, hence a rich running engine.

Oxygen sensors usually put out a very small voltage when measuring oxygen content. Typically, they only put out from 0 to 1.1 volts max. 

     All internal combustion engines need a proper air-fuel ratio in order to run correctly. Gasoline engines need to run a ratio of 14.7 parts of air to one part of fuel. (Air cooled motors typically need to be slightly richer).

     When an engine has more fuel than it needs, all the oxygen in the cylinder head is consumed and the resulting exhaust gases contain almost no oxygen. This usually causes the sensor to generate a voltage of greater than 0.45 volts. If the engine is running too lean, all the fuel is burned, and the extra oxygen remaining flows out into the exhaust manifold. The oxygen sensor detects the extra oxygen and the voltage signal will drop to below 0.45 volts.. 0.45 volts is usually the voltage where the car is neither running rich nor lean, this is when the car’s mixture is usually perfect.

     Oxygen sensors usually generate an output voltage between 0.2 to 0.7 volts. However, Oxygen sensors doe not begin to generate full output voltage until it reaches about 600 degrees F. Some oxygen sensors have a built-in heating element that lets the sensor heat up quicker.  Keep in mind that most oxygen sensors work in an open loop until they have reached a certain temperature. What this means is that until it has heated up, the car will not use the oxygen sensor to measure the mixture, but rather use all the other sensors on the car. The sensor will generate a constant 0.45 volts until it has heated up.  Once the sensor reaches a certain temperature, it becomes a closed loop and the sensor starts to generate variable voltage.

How do I test the sensor?

     The first step is to let the car warm up to operating temperature. You will need a high-impedance DC voltmeter to measure the output voltage. It’s a good idea to use a high quality or digital voltmeter. Analog voltmeters usually are not sensitive enough to register the small voltages generated by the oxygen sensor.

       The first step is to get the engine warmed up to operating temperature. This insures that the oxygen sensor will generate voltage. Now, attach the positive lead of the voltmeter to the oxygen sensor output wire.

     This wire should remain connected to the harness going to the computer, so you may find it necessary to use a jumper or trim back the insulation so you can attach the leads. Connect the negative lead to a good engine ground, such as the engine block, or any bare metal on the vehicle’s chassis. Now, set the voltmeter to look for 1 volt DC.  When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check the connections.

     Now start the engine.  In the case of single wire sensors, you should run the engine above 2000 rpm for a few minutes to heat up the O2 sensor and try to get into closed loop. The sensor showing several cross counts per second indicates closed loop operation. It may help to rev the engine between idle and about 3000 rpm several times. The computer will recognize the sensor as hot and active once there are several cross counts.

     You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high, near 0.45 or steady low?  If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of its hose    or remove the oil filler cap and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good. If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor. If you are not getting a voltage and the car has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down. The extra heat will clean it off if possible. If not, the sensor is fried. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.

My car has two (or more) wires coming out of the sensor, which wire is the signal output? 

     Most modern cars used a heated oxygen sensor. In this case, there will be two three or four wires instead of one. What the heating element does is heat the oxygen sensor up quicker than simply relying on the heat of the exhaust manifold.

     If you have two, three, or four wires, use a 15 or higher volt scale on the meter to test each wire for 12 volts. You will need to determine which wire is the signal output wire. Once you have determined which wire is the output signal wire, test the sensor as shown above. 

     A heated oxygen sensor with two wires is usually wired like this (pic o2_sensor_diagram_2.jpg) One wire is 12 volts for the heating element. The other wire is the signal output wire to the fuel injection computer.  In this case, the oxygen sensor casing is the ground.

     With a 3 wire oxygen sensor, it is wired like this, (pic  o2_sensor_diagram_3.jpg) one wire is 12 volts for the heating element, one is ground for the heating element, and the last wire is the signal output to the fuel injection computer.

     Lastly, with a 4-wire oxygen sensor, the output signal works in a constant loop, essentially, the fuel injection computer sends a signal to the sensor, and the sensor then sends the signal back to the computer. This is achieved by having one wire carry the signal to the sensor, and then another wire carries the signal back to the fuel injection computer. You also have a wire carrying 12 volts to the heating element, and the last wire is the ground for the heating element. (pic  o2_sensor_diagram_4 jpg) With a 4-wire sensor, you will need to measure the voltage fluctuations between the two signal wires. To do this, start the car and let it warm up. Next, disconnect the oxygen sensor and measure the voltage between the wires. Now take off the oil filler cap to simulate a rich running condition. Watch the voltage, if it begins to fluctuate rapidly, then the sensor is good and you’re done. If it stays the same, it’s probably fried.

     Well, there you have it - it's really not too difficult at all.  If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs.  If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one.  Your continued support directly affects the expansion and existence of this site and technical articles like this one.  As always, if you have any questions or comments about this helpful article, please drop us a line.

Comments and Suggestions:
SlimmComments: I have a 1994 Nissan Altima GXE and a few days ago it started wanting to go dead and shake when I shifted to 1st gear. Then in gears 2-4 it doesn't want to accelerate properly and in 5th in shakes so bad I couldn't use it. Then this morning 1st thru 4th gear work fine with proper acceleration but 5th gear still shakes but not as bad as before. I was told that the other day it was possible the Oxygen Sensor bad but now I not too sure. Is it the Oxygen sensor or something else?

Thanks a lot
April 15, 2014
 Followup from the Pelican Staff: Sounds like an engine misfire. Check each cylinder for spark, fuel and compression. - Nick at Pelican Parts  
JJ3Comments: Nick ,How long should you run a 2000 GMC Sierra 1500 with 4.3 ltr v6 after replacing the O2 sensors before the computer will show ready when doing emmissions test ??
April 1, 2014
 Followup from the Pelican Staff: Should not take long. Maybe 30 minutes. - Nick at Pelican Parts  
LorettaComments: We bought a 2005 doge grand caravan and took it to get inspected and they said we had to change the oxygen sensor bc of the check engine light. We drove it over 50 miles to clear it and now we wake up this morning and the light is back on. Can you please tell me what to do?
March 30, 2014
 Followup from the Pelican Staff: I would go with the recommendation of your mechanic and repair the faulty sensor. - Nick at Pelican Parts  
DannyComments: Hello,On a 1999 Infiniti I30,if the H02S1-B2 Front sensor and the HO2S2-B1 rear sensors were reversed at installation would make car run noisly and stall, how can I test for this.
March 27, 2014
 Followup from the Pelican Staff: You can check the signals using a scan tool. Then check if the fuel trim seems flipped. if the o2 is doing opposite of the fuel trim, and the fuel trim is locked in one position, they may be swapped. - Nick at Pelican Parts  
RomanComments: I changed the o2 sensor on my 2000 honda civic ex 3 times and it still reads p0135 bank one sensor one I checked all my fuses and the light still won't turn off ? What else could it be !!? Please help
March 25, 2014
 Followup from the Pelican Staff: Could be the wiring. - Nick at Pelican Parts  
StanComments: Just had a new flex pipe installed on my 2005 Hyundai SUV. I now have an odor of burned oil coming into the car. I was told these pipes have a coating for rust on them and it probably is burning off. The mechanic checked for faulty sensors but only a history code came up vs. an active code. The check engine light hasn't come back on. Do you know if the smell could be coming from this flex pipe. Mechanic says the exhaust system is good.
March 23, 2014
 Followup from the Pelican Staff: New pipes do emit an odor for a few miles. If it doesn't go away, go back to the shop. - Nick at Pelican Parts  
NisaComments: I have a 2006 CTS. I just had the fuel injector serviced and replaced a part that wasn't working properly. a part that liquid supposed to be coming out of one manifold, but was coming out of both ports, after doing this my check engine light came on. I took it back, the mechanic reset it and said it was the car burning off the injector fluid. It has came on twice since then, I took it to AutoZone and had them put it on the check engine light machine. He said it was the fuel sensor. But the this wasn't happening before I got the service on the fuel injector. An help, please.
March 20, 2014
 Followup from the Pelican Staff: If there is a fuel leak, I would have it repaired immediately and advise you against driving it. Have the shop explain and show you what they mean by fluid. This is your best bet. - Nick at Pelican Parts  
brianComments: I have a 97 dodge 1500 5.2 2wd replaced transmission and now its running rough? If overfull will it not go into gear or. Whatsif o2 sensors are crossed
March 17, 2014
 Followup from the Pelican Staff: Is the engine misfiring? What parts did you replace along with the trans? - Nick at Pelican Parts  

press reset button for inertia swith.....preacher xjs 3/3/14

March 6, 2014
 Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help.
- Nick at Pelican Parts
TeeksyComments: I have a 2003 dodge Durango with codes p0172 and p0175 system too rich bank 1 and bank 2.. Car idles rough when at a stop for a long time but drives fine. Also stalls as if it doesn't want to drive when I step on the gas from a complete stop. was having these problems before the codes piped up it was first the Throttle position Sensor. NEED HELP FINDIG A SOLUTION don't want to keep paying for what people think it could be instead of the actual problem?
March 5, 2014
 Followup from the Pelican Staff: I would use a Dodge scan tool and confirm the vehicle fuel trim is rich and when it is rich. You could have a faulty component causing a rich condition or a false rich condition. - Nick at Pelican Parts  

March 3, 2014
 Followup from the Pelican Staff: Hyperjam had this to say:

press reset button for inertia switch- Nick at Pelican Parts
preacherComments: great info.
March 3, 2014
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
kimmieComments: Well that's what my car be doing only way i will have power and that's if i i take my foot off acceleration
February 27, 2014
 Followup from the Pelican Staff: Sounds like you are in the right area then. - Nick at Pelican Parts  
kimmieComments: What is some signs of transmission slipping
February 27, 2014
 Followup from the Pelican Staff: Attempting to accelerate, RPMs will go up, vehicle speed will not follow, feeling of lower power. - Nick at Pelican Parts  
kimmieComments: What can i use if transmission is slipping
February 27, 2014
 Followup from the Pelican Staff: A new transmission. - Nick at Pelican Parts  
February 26, 2014
 Followup from the Pelican Staff: Could be a the transmission slipping. - Nick at Pelican Parts  
grampsbComments: 2000 mercury villager 3.3v6. runs great cold but when it warms up, runs rough. new cap and rotor, wires and plugs. the code is for multi-misfires I think it p3000. also the converter will glow red when the motor warms up.
February 24, 2014
 Followup from the Pelican Staff: If the cat is glowing, there is a misfire. Check for good spark, fuel and compression on each cylinder. - Nick at Pelican Parts  
formula oneComments: the case of the car‘s engine consists of 6 cylinder and lost the signal of O2 sensor 1 bank 1 ,how will the computer to process and calculate the ratio of air and fuel mixture to both bank 1 and 2 .
February 13, 2014
 Followup from the Pelican Staff: The DME will run in pen loop if an o2 sensor is lost. This is a programmed fuel map. - Nick at Pelican Parts  
formula oneComments: Dir Sir , it is very useful and I thank you for this information.
God bless you.
Whit my sincere respect and appreciation
February 2, 2014
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
HiuszyComments: Hey I have a 1993 chev truck, and it recently became crappy,
Its going through tons of fuel, its lost a ton of power, revs really
high before switching from first to second, or second to third,
And won't go into fourth or fifthod gear and very low top speed
Any thoughts? Please email as soon as read please and thank you
January 29, 2014
 Followup from the Pelican Staff: Could be an engine misfire. I would check the engine control module for fault codes. - Nick at Pelican Parts  
4runner owner Comments: 2003 4runner 4WD 6 cyl. would start but not turn over. fuel pump replaced and ck engine light kept coming on and engine dragging. replaced o2 sensors no light yet but engine is dragging still. mechanics stumped. Need help.
December 28, 2013
 Followup from the Pelican Staff: I can't help without knowing what is missing from the system when you are trying to start it. When your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector pulse and pressure fuel, volume and quality. Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
T4 primeraComments: hey Nick I have a Nissan Primera t4series p11 and I have been getting a pinging sound at 1500 rpm.the car starts fin idles smooth accelerates very good,full power,i removed all the heat shield from the exhaust thinking it was a vibrating sound .but then the pinging soung is still there,i diagnosed the car and everything is perfect.any suggestions.
December 28, 2013
 Followup from the Pelican Staff: If everything is perfect I fail to see the problem. ;)

I suggest isolating the area the noise is coming form. At times this can be difficult. Does it sound like its coming from inside the engine or from an engine accessory? is it from the transmission area?

- Nick at Pelican Parts
richComments: 2006 Mazda 6 V6. 210K. Was experiencing slight smell, but no check engine light and car seemed to run fine. Had taken to shop to get plugs changed. A few days later got into town and after I came to stop car would rev but not move. Pulled it over and left it. Came back to get it and it started fine and drove off, but when I came to a stop sign a few minutes later it had same problem when I went to accelerate. Is this a sign of a very dirty/faulty O2 sensor?
December 21, 2013
 Followup from the Pelican Staff: An oxygen sensor will not cause a not start.

I can't help without knowing what is missing from the system when you are trying to start it. When your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector pulse and pressure fuel, volume and quality. Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
brettComments: hi I have a 2001 dodge neon r/t and I bout a after market downpipe and I didn't have o2 sensors on it for a while and it started to run rough it will cut out at 4000 rpms I put the old o2 sensors on the new downpipe and I cleared the o2 sensor codes and still runs rough.
December 21, 2013
 Followup from the Pelican Staff: I would start by checking the engine for faults and perform an engine balance test. - Nick at Pelican Parts  
shannenComments: my 2004 es 330 lexus starts losing power when I try to push down on the throttle more than half way. Any ideas what it could be?
December 16, 2013
 Followup from the Pelican Staff: Check if the exhaust is restricted. Test backpressure. - Nick at Pelican Parts  
LunetComments: Lots of good info on your article here! We have a 2005 Nissan Sentra w/ 75000 miles on it. Since July when we would start it cold it would run rough for about 10-20 seconds then even out without any other problems. Recently it started smelling like gas on start up as well. When they check OBD the check engine light goes off and no codes are given even though it's been on the rest of the time. One mechanic said cam sensor or throttle position. Could it instead be the Oxygen sensor? Thanks for your time! :
December 15, 2013
 Followup from the Pelican Staff: If you smell fuel, i would check the evap system. You could have a leak, you would smell vapors, then it would turn into a possible vacuum leak. - Nick at Pelican Parts  
SamComments: Hello again
can you use planar Zerconia narrow band o2 sensor for cars that were originally fitted with Zerconia thimble narrow band ?. Considering that it is only the heater element is different ,and the planar requires less power anyway .
December 13, 2013
 Followup from the Pelican Staff: No, a narrow band has to be used as the computer is fitted to work with it. - Nick at Pelican Parts  
SamComments: Hi
ref : the 4 wire O2 sensors

You wrote " wire carry the signal to the sensor, and then another wire carries the signal back to the fuel injection computer"

. I thought the 4 wires of the 4 wire sensors are as follows : 2 for heating element , one for the ground , and the fourth wire is the signal output .!

how could the ecu sends voltage to the sensor through the ground wire ?. I would be grateful if you could clarify .

and as for testing the 4 wire sensor, every article I have seen calls for connecting the positive lead of the voltmeter to the signal wire , and the negative lead of the voltmeter to the ground and watch for the voltage etc ..

I would be really grateful if you could clarify .
many thanks and kindest of regards

December 12, 2013
 Followup from the Pelican Staff: Some sensors use a floating ground.
However, on most narrow band, the DME sends a 450mv ref voltage to the sensor for comprehensive component monitoring. - Nick at Pelican Parts
Tom MComments: I have an 03 525i. I replaced both front o2 sensors and also had the car at the dealer where they fixed a evap league. My mileage improved after the above repairs. The car runs fine but I continue to get a check engine light and the codes from Peake reader are E10 and E11 which are post cat sensors. Additionally, the check engine light sometimes goes off while driving before it is reset. I have read that the post cat 02 sensors don't send a bad code, rather it is the cat itself. Can you tell me if I need to replace the post cat 02 sensors or is it my cat? Also, where are the post cat sensors? I assumed they would be in the cat but I don't see them. Thanks
December 9, 2013
 Followup from the Pelican Staff: Likely not the sensors, the codes don't come up for me, but it could be faulty cats. - Nick at Pelican Parts  
KayceeComments: I have a 2007 Jeep Compass. We recently had our mechanic replace our O2 sensors. Now the car is slow to accelerate and makes a sort of clicking sound when in motion. This wasn't occurring before the sensors were replaced. Do you think it has something to do with when the work was done?
December 7, 2013
 Followup from the Pelican Staff: No telling. You'll have to take it back to the folks who worked ion it and explain the situation. - Nick at Pelican Parts  
frankComments: I have a 99 toyota avalon. When the car is running and parked or at a stand still it smells like something is burning. When I try accelerating it feels like the engine is putting. Anynidea what it could be
November 29, 2013
 Followup from the Pelican Staff: There are too many possibilities with the description you provide--anything from a plastic bag stuck to the muffler under the vehicle to a misfire due to engine defects. - Nick at Pelican Parts  
deeComments: I have a 01 gs 300 lexus.Early in the morning and after sitting for a while,I have a high idle for about 5 to 7mins then it runs fine.Could it be a bad o2 senor?
November 23, 2013
 Followup from the Pelican Staff: I would start by checking for vacuum leaks. - Nick at Pelican Parts  
JamesComments: Hi, I have a 2001 Toyota Corolla. My mechanic changed my O2 sensors after my engine light came on. But since then, my car stutters and doesn't accelerate properly; it won't accelerate from standing still unless I slam several times, sometimes the gas pedal, then it jumps several 1000 rpms & lurches forward. What could be the issues with my car?
November 22, 2013
 Followup from the Pelican Staff: It is possible the wrong sensors were installed. Especially if the fault was not present before. I would double check the part numbers against your VIN. - Nick at Pelican Parts  
lexComments: Thanks Nick for the reply, I thankfully did, it was the piston rings that had shattered, its having a new engine under warranty! ! And ive also asked for the cat to be checked at the same time incase its been damaged :
November 13, 2013
 Followup from the Pelican Staff: Good call, glad it worked out, and thanks for the follow up. - Nick at Pelican Parts  
TalmafComments: Hi Nick I have a Honda Civic 2003, recently it started misfiring with rough idle, and power loss. I took it up for diagnosis and the test revealed misfiring cylinder number 4 and HO2S B2 S2 heater circuit mulfunction. Would a bad O2 sensor cause a misfire or the other way round? Also how bad is it too keep driving it like that till I get the money to get it fixed?
November 13, 2013
 Followup from the Pelican Staff: An oxygen sensor will not cause a misfire, it can create drivability issue. If your engine is misfiring I would deal with it asap before the catalyst is damaged. This could increase the cost of repair it if happens. - Nick at Pelican Parts  
lexieComments: I hace recently bought 2005 toyota corolla vvti 1.6, after doing a round trip of approx 120miles the engine management light has come on and the oil light flicked on twice then went off, the vehicle then went into limp mode. Taken it to the garage as under warranty, they pulled the code and said something about an air leak, however once they spoke to the dealer I bought it from they miraculously wiped the code without noting it down, told me it just needed oil and fobbed me off!! Car has reduced performance and doesnt go over 80mph in 5th gear with pedal to the metal and tge engine sounds like its at full capacity with rpm staying over 4000!!!! Could this be an o2 sensor, I have managed to get off the garage that the code said something about catalyst emission below threshold? ?? They wiped the code so cant get independent garafe to check now! Any suggestions appreciated as taking it back to them tomorrow and not be fobbed off again! Thanks
November 11, 2013
 Followup from the Pelican Staff: This could be a faulty or restricted cat. I would take it to the dealer who honors the warranty and have it looked at. - Nick at Pelican Parts  
YeyoComments: I have a 1998 Cheyenne truck. They scanned and the problem seemed to be the starter relay, We replaced it and it better up for a moment,just a day later it didn't start again. I cleaned up all the relays and the fuse box with an SQ spray electronic cleanerand it started again, I shut it off a few times and started back with no problem at all Next day I try to start it ant it didn't even iddle at all Would You help me please I will appreciate Than You
November 10, 2013
 Followup from the Pelican Staff: I would check if there is a problem with the relay panel. Maybe corrosion that you are temporarily remedying when spraying it. - Nick at Pelican Parts  
Desiree NashComments: 2002 audi a4 keeps throwing code p0421 & p0431. I've replace the spark plugs and vac hoses. Do u think its the 02 sensors?
October 30, 2013
 Followup from the Pelican Staff: those codes are for the cats, It sounds like both of the cats need to be replaced.
- Nick at Pelican Parts
Hot rodComments: hello i have a 1997 sabb 900 se turbo the check engine light was on for crank sensor And coolant level sensor and a oxygen sensor.We replaced all new.Now the check engine light is back on saying bank #1 sensor 1.So,I put a another oxygen sensor in bank 1 sensor 1 in check engine light come back on saying bank 1 sensor 1.And failed smog test.What can I do?And can I check the manafold air pressure?Do you know the specks and a hand held scanner and fuel pressure specks?
October 25, 2013
 Followup from the Pelican Staff: I would start by checking the o2 sensor signal with a volt meter and see if it changes rich to lean. If if is stuck on the lean side start looking for a vacuum leak, if it is stuck on the rich side, start looking for a leaking injector or a fuel pressure problem. The fuel pressure should be 3 bar.
- Nick at Pelican Parts
angelComments: I have a 1996 Nissan Altima gxe my problem is the car cranksup and runs good but when it decides to cut off it does with no notice,We can wait a few minutes and it will crank right backup with no problem and then it might run a few minutes with no problem and then it will quit again.Could you please give me an idea what it might be.We have already took it to a shop and they have no clue what it could be
October 23, 2013
 Followup from the Pelican Staff: It could be a sensor or a fuel problem, is the check engine light on? Is there fuel pressure when it won't start?
- Nick at Pelican Parts
JessieJComments: Hi, I have a auto 2001 Nissan Primera stationwagon. I have just had the o2 sensor replaced. However when taking off at intersections or when in lower gears and attempting to get up into the higher gears and speed up. When I put the foot down, instead of acceleration, I get a roar from the engine and the rev counter goes way up. I usually ease off on the accelerator and gear down and gear up again more slowly and generally get to the desired speed but it is not the way this car normally reacts. I am at a loss as to what the problem maybe.
October 14, 2013
 Followup from the Pelican Staff: Sounds to me like the transmission is worn out. First check the fluid level to be sure it is correct.
- Nick at Pelican Parts
HeriComments: BMW 1994-530I. I installed new original bosch O2 sensors and the car start running perfect. One week later the car lost power and engine is shaking again. I check the codes and say bad O2 sensors. I clean them with brake cleaner and small amound of dust came out of the sensors. Put them back and car is running perfect again. What is causing the sensors to get dirty in one week? Are the Catalytic's Plugs?
October 8, 2013
 Followup from the Pelican Staff: You need to check the engine to see if it is running too rich or lean because of a vacuum leak or a fuel system problem. Also measure the backpressure on the cat and see if it is plugged.
- Nick at Pelican Parts
thorpyComments: ivw got an x type jag i have put a new oxygen sensor on it and had fault code erased but engine management light has now come back on and its the same fault code any ideas as to what is causing this
October 7, 2013
 Followup from the Pelican Staff: What sensor has failed, what is the fault?
- Nick at Pelican Parts
SuperTruckComments: I have a 2003 Dodge Durango and the o2 sensor has gone bad and been replaced multiple times. The shop I took it to said that it needed to be replaced each time but once it gets replaced it comes with the same code and check engine light comes on once I drive it and then turn it off and start up again. Same codes are being shot out once the check engine light comes on. Shop suggested that I need a new computer to solve this and it may not be the o2 Sensor at all. Please help
October 2, 2013
 Followup from the Pelican Staff: What sensor has failed? What is the fault?
- Nick at Pelican Parts
manshaComments: most of the answers are correct
September 29, 2013
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
RandyComments: Code for bank 1 sensor 2 keeps coming up even after changing sensor. I got another from dealer with same results. What could be wrong??
September 23, 2013
 Followup from the Pelican Staff: You could have a problem with the way the engine is running causing the sensor to never switch from rich to lean, when the DME sees this it thinks the sensor is faulty. Check for a leaking injector or a vacuum leak. Use a meter to check the voltage signal from the O2 sensor to see where it is, try to make it react by forcing the engine rich or lean. If you can force the sensor one way or the other then you know it works correctly and you have to fix why the engine is not running correctly.
- Nick at Pelican Parts
philComments: I have a 2010 Volkswagen turbo diesel.It has 21,000 miles on it and the oxygen sensor had to be replaced.What would have caused this failure?
September 17, 2013
 Followup from the Pelican Staff: The sensors can fail, sometimes it is just a defective part.
- Nick at Pelican Parts
DickyComments: Rebuilt top end and water pump in a 2004 Dodge ran ok before, now it wants to fire maybe every 4th turn over but won't start. I can't find any loose connections and it's getting fuel. Any ideas?
August 30, 2013
 Followup from the Pelican Staff:
If your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector pulse and fuel pressure, volume and quality. Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
SueComments: we replaced both bank 1 and bank 2 oxygen sensors on our Lexus RX300 2000 and our car started shifting hard between 20 mph and then again near 30 mph, once the oxygen sensors are removed the car shifts fine! There are no codes either showing what is wrong???
July 11, 2013
 Followup from the Pelican Staff: When you change O2 sensors you need to clear the PCM adaptations. You can do this on your SUV by disconnecting the battery for 5 minutes - Nick at Pelican Parts  
bicyComments: I have a 99 kia sportage I can drive it 10 mins n cut it off n it starts back up but when I drive it across town I cut it off then try starting it it sounds like it needs a jump I replace the battery n the starter so is it because something is getting too hot where I would have to sit n wait till it cool off because I can let it sit for a while n then it will start back up with no problem
June 27, 2013
 Followup from the Pelican Staff: If your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector pulse and fuel pressure, volume and quality. Once you figure out what is missing, it will be easier to diagnose. - Nick at Pelican Parts  
FaridComments: Hi,I have caprice Ls 89.The previous owner of the car disconnected O2 sensor.After one year of driving I found that the car has no O2 sensor.All these times there was no engine light on and no code.Why it is so?without o2 sensor fuel consumption was 14 lits per 100 km.
May 3, 2013
 Followup from the Pelican Staff: Is the vehicle a 1989? Then it may not set a check engine light, depends on what parameters it takes to do so. There could also be o2 simulators installed. - Nick at Pelican Parts  
The MaverickComments: Hi Nick, It is wonderful to know that one can do all these technologically advanced searches on an engine to find problems such as O2 lean settings etc. We are however stuck in a small 3rd world African country called Namibia or previously known as South West Africa where such advances in technology are few and far between. One can get them for the more expensive brands such as BMW and Mercedes and all the later models,, but for the old 2001 KIA Sportage with it's antiquated Mazda FE3N engine,, I am not even going to go to any further expense. I will grin and bear it for another few days until the Toyota engine is ready and then some other guy with more patience and money can have the engine to fool around with. The new under hood wiring harness is ready today and the bell housing will be ready on Friday and then all I need are the engine mountings. Then yippee,,, I can go play around in the desert again without fear of running out of power or fuel.. Thanks for your input. It is much appreciated.
April 23, 2013
 Followup from the Pelican Staff: No problem, glad to help. - Nick at Pelican Parts  
The MaverickComments: Hi Nick,, I have all but given up on the shop which did the original work. They are responsible for the engine overheating and requiring a rebuild in the first place by installing the fan the wrong way around. I have refused to pay them for their original error. We only communicate via our lawyers. I have in the meantime sourced a Toyota 2.4 engine in very good condition and low kilo's and will be preparing this to install in the Kia in the next few weeks. All I need to do is have a bell housing conversion made and then it will read on the spare wheel cover "Reliably powered by Toyota" !! My previous Toyota 2.4 Hilux pickup had over 400 thousand Kilos on the clock before I sold it. It also never let me down and was rather economical on fuel for a 4 x 4 which is more I can say for the Kia !!! No O2 sensors there to give me trouble... Fool me once,, shame on you,,, fool me twice,, shame on me !!
April 23, 2013
 Followup from the Pelican Staff: before you stick the fork in the engine. See if you can put a scanner on it. Check for an o2 hanging lean, or something that could force a rich condition. Good luck! - Nick at Pelican Parts  
The MaverickComments: I drive a 2001 Kia Sportage 2.0 DOHC 4 x 4 and I am experiencing terrible fuel consumption.. The engine was entirely rebuilt after a bad overheating episode and the airflow sensor as well as the Oxygen Lamda sensor at the manifold were replaced. The car ran fine for one or two days after these replacements but has now reverted back to very heavy fuel consumption... What can be the problem and where can I start looking for problems. All hoses were replaced and the job was professionally done and I am about ready to park it on a deserted road with a full tank and the keys in the ignition in the hope that someone will relieve me of it and its problems !!! It emits a black smoke like a diesel, but after testing all sensors,, everything seems fine and working,,,, yet it continues to bankrupt me with fuel consumption !!! Please send help to
April 19, 2013
 Followup from the Pelican Staff: If you tested everything and it seems fine, yet vehicle smokes badly, you have missed something. I would return to the shop that installed then engine and ask their advice on the matter. - Nick at Pelican Parts  
ada Comments: i have a e36 1994 can someone help me my exhust emissions it fail it mot i have put on a new lambda the fast idle ane second fast idle test
March 18, 2013
 Followup from the Pelican Staff: What gases failed? Can you share the tailpipe emission readout of all gases? - Nick at Pelican Parts  
janelleComments: I have a 1996 nercury villager
van that when trying to accelerate it goes slow then jerks can u help with what it could be? greatly apperciated.
March 12, 2013
 Followup from the Pelican Staff: Sounds like you have a hesitation. Check the engine control module for fault codes. - Nick at Pelican Parts  
evoComments: Hi, can anyone help me. I have a 2001 saxo vtr, I have just noticed it has 2 lamba o2 sensors for the ehaust but only 1 is plugged in an the other is rapped up an pushed to the side, this is how I braught It so I don't no if its had a different exhaust fitted wich only allows 1 02 sensor, my engine management light is on an is using quite abit of fuel, what can I do, can I wire them both up into 1 sensor. Thank you
February 13, 2013
 Followup from the Pelican Staff: Sorry, I am not familiar with your vehicvle. We do not have it here in the US. - Nick at Pelican Parts  
ZWolf24Comments: Hello many, I can't seem to figure out which wires are which and no multimeter, but i hope its an easy once the pic is seen. Outside wires are white, inside right wire black, inside left gray? Just hoping to know which is which if possible. Thank you.
February 1, 2013
 Followup from the Pelican Staff: I'm not sure which car you referring to here. I will copy this question forms and perhaps you can add that information there and we can help you out. - Wayne at Pelican Parts  
JohnWComments: Hi, I have 2001 e39 with 135000 miles, got SES light on and Peake code shows 19 96 . I change 2 new pre-cat O2 sensors. Reset fault code but still got SES light on after restart engine. Please help
January 3, 2013
 Followup from the Pelican Staff: The CEL light can also come on if there are faults stored in the trans control unit. Have the vehicle scanned with a factory scan tool to see what codes are present.
- Nick at Pelican Parts
samcComments: My local mechanic thinks my theory is full of hot air. I've had my 98 540i for over six years and had 3 DEQ tests in Oregon. The computer does it's thing and the car passes. The last two times I've done that the check engine soon light comes on within 2 miles of leaving and the problem both times has been an O2 sensor. Am I just randomly unlucky ?
October 24, 2012
 Followup from the Pelican Staff: Just running an OBD2 emissions test should not cause any faults in the O2 sensors. I think the sensors just failed.
- Nick at Pelican Parts
TrishComments: We have a dodge caravan 2000 that we bought from a tow yard. We have had the vehicle for several years and it has done us really well until.recently. when driving in warmer weather at a higher altitude .about 4000u plus.. it will drop mileage and then shut down. If u unplug the battery and reset the computer sit for about 10 min then rehook will run fine for rest of time driving.. could this be an o2 sensor?
July 10, 2012
 Followup from the Pelican Staff: Unlikely it is the o2 sensor. What may be happening is the vehicle sets a barometric pressure spec when key is turned ON, then you go up in altitude and things get skewed. Cycling the key resets the baro reading and allows to you to drive again. - Nick at Pelican Parts  
GerryComments: Great article, thanks, my '89 E34 535i just started running rough after I refuelled. Thinking I had a dose of crook fuel, I quickly replaced the fuel filter. Did not help. Symptoms are, smooth initially, then if throttle depressed any more than about 15%, she starts to miss, buck and finally loose power. Most disconcerting for such a normally silky smooth engine. Fuel consumption has now risen to 26.4 litres/100km, eeeek! 02 sensor hopefully will solve the issue, as I can't justify a smoke generator. Cheers all from Australia, cya.
June 17, 2012
 Followup from the Pelican Staff: I would measure the fuel pressure and the volume, pressure should be 45 psi and the volume should be 30 oz in 30 seconds. - Nick at Pelican Parts  
ScottComments: The article about oxygen sensors had just the right amount of technical language. It was well written and clear, and I could follow it perfectly. Thank you so much for a very informative and helpful explanation!
March 15, 2012
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
LimcmengComments: Hi I have been experiencing drop in engine revsmore than usual immediately after a gear change only when the engine is not fully warmed up yet. Can this be the oxygen sensor issue?
March 14, 2012
 Followup from the Pelican Staff: I would take a look at the idle valve and check the DME for faults. - Nick at Pelican Parts  
ThomasComments: Oxygen sensors generate absolutely NO voltage when cold. They do not generate a steady .45 volts. That is a voltage produced or derived, since it actually comes from the vehicle's power system within the computer and is fed through a very high resistance to the O2 sensor circuit. This tells the computer that the O2 sensor is not producing its own voltage. Once the O2 sensor is heated up, it will override this voltage. It will both drive the voltage as high as one volt and be able to pull the voltage all the way down to zero volts. If you disconnect the O2 sensor, you will see the .45 volts generated by the computer, which proves that it is not generated by the sensor.
March 6, 2012
 Followup from the Pelican Staff: Thanks for the additional Info. We appreciate it.
- Nick at Pelican Parts
BMWinSDComments: I have a 96 BMW 740iL that is getting error codes 0156, 1186 and 1187. I looked up what those codes mean, and they seem to be due to issues with the post cat O2 sensor. Does this mean that those sensors need to be replaced, or are there other things that can be done to eliminate those codes? I am assuming that those codes are not preventive. They keep coming back after they are erased.
December 21, 2011
 Followup from the Pelican Staff: The codes are for the o2 sensor heaters, make sure the sensors are getting power to the heater circuit, if so replace the sensors.
- Nick at Pelican Parts
MetsadaComments: I have a new 2011 bmw X3 35i for 3days now. Many people said its fuel consumption about 19-22mpg somehow my car is only 11mpg, is there something wrong with my car?
December 15, 2011
 Followup from the Pelican Staff: The MPG on a new car will be lower until the motor breaks in, it will also be affected by how and where you drive. Drive the car hard and the MPG will be less, stop and go traffic will also lower MPG.
- Nick at Pelican Parts
SpookydragonComments: Hi, I have a 95 Audi cab 2.6 V6 ABC engine, it has a three wire zirconia sensor on each engine manifold, 2 white heater wires and a black sensor wire, the right bank sensor tested OK but the other one gives me 12 volts on the sensor wire, they're not broke or crossed and there's only one way to connect up, what gives ?
November 30, 2011
 Followup from the Pelican Staff: I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
man55Comments: mercedes ml320 petrol 1999 engine starts first time avery time but will not rev up mote then 2500 rpm if accelarated over that it atart missing and drop the rev to 900rpm and start missing
November 24, 2011
 Followup from the Pelican Staff: I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
chrisComments: could a bad sensor make your engine overheat? yes, but only if the engine is badly undercooled to begin with. could a bad sensor make one cylinder misfire? conceivably, but not likely. the fact is a misfiring cylinder will make the sensor read lean no matter what the ecu does, so it will report a bad sensor if it's not too smart. i would test the sensor in a standard rederence cell, look for another cause to the misfiring, and go from there.
November 24, 2011
 Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help.
- Nick at Pelican Parts
AlexComments: Is there a way to clean the soot/dirty at the tip of the sensor. My car had a bad airflow meter and was oozing black smoke like diesel engine yet its petrol. Did this not affect the O2 sensor and do I need a new one or the is way of cleaning as we do for spark plugs????
October 21, 2011
 Followup from the Pelican Staff: The sensor should be fine, it will clean itself out while the engine is running correctly.
- Nick at Pelican Parts
AdeelComments: Hi, I want to ask problem in oxygen sensor always lightens up the check engine light or not???
October 13, 2011
 Followup from the Pelican Staff: If there a fault in the system the light should come on.
- Nick at Pelican Parts
jukoComments: check engine light on,engine almost or even switches of especially when on drive gear and you step on diagnostic shows trouble code p0172 fuel sys-rich bk1.what does it mean and whats the solution?thankscheck engine light on,engine almost or even switches of especially when on drive gear and you step on diagnostic shows trouble code p0172 fuel sys-rich bk1.what does it mean and whats the solution?thanks
August 25, 2011
 Followup from the Pelican Staff: It means that the DME can no longer add fuel and has reaced its limit, look for a vacuum leak, or a faulty fuel injector.
- Nick at Pelican Parts
RussComments: Hi thanks for your advice.....i took the car back and a fault code read "Converter lock out - excessive slipping....the garage replaced the new second hand ! converter with a recon one....problem is exactly the same....all been checked for air leaks and there are none...the garage seem to be at a loss...any help......please
April 20, 2011
 Followup from the Pelican Staff: I would check the converter lock up solenoid.
- Nick at Pelican Parts
RussComments: I ve just had the torque converter replaced on my BMW 330CI. Since getting the car back ive noticed that the car seems to be running sluggish. On the motorway if iam driving along around 60 - 70 MPH and i put my foot easy on the accelerator there seems to be no response. The petrol gauge goes up but the rev counter stands still. Eventually the car gains pace and still drives superb at higher speeds or when i kickdown. There have been two codes recently pointing to the o2 sensor post cat bank 2 but i had one of these changed a couple of months ago. At the moment there are no warning lights on...iam hoping theres some sort of air leak and its not a transmission problem starts fine and idles fine changed the plugs but no change...any thoughts....
March 28, 2011
 Followup from the Pelican Staff: Hmm, if you've had the torque converter replaced recently, then I would take a closer look at that being the problem. On the freeway when you step on the gas, the engine will try to push the car faster through the transmission, which includes the torque converter. If there's something wrong with the converter, then I think you might see some of the issues that you're thinking you see. I would take it back to them and have them read the codes and drive the car for a while to see if there are any issues. - Wayne at Pelican Parts  
mmm3Comments: Wayne, I have recently purchased 2001 e46 m3 and I do not think it is producing enough power compaired to another one I drove a year ago. The Vanos unit was replaced at the dealer some time ago, do you think I should check the cam timing?? incase they stuffed up? or any other ideas??? apart from that all service items seem to be up to date.
March 24, 2011
 Followup from the Pelican Staff: It's impossible to tell and compare to a car you drove a year ago. If you think that something is amiss with the car, then pull the codes and see if there are any problems. If you think the cam timing is off, then you can plug it into the computer and read it and check it against the spec. - Wayne at Pelican Parts  
nissan kingComments: is it possible for the converter to be clogging or clogged already and causing more codes to show on a scanner? if so what are other things that may happen? Thanks!!!!n
March 16, 2011
 Followup from the Pelican Staff: Yes, it is possible for the catalytic converter to be clogged - that would typically show up as a post-cat O2 sensor error. The secondary oxygen sensors are installed in order to monitor the performance of the catalytic converters. If they don't read what they are expecting to read (based upon the values from the pre-cat sensors), then they will trigger fuel injection error codes. - Wayne at Pelican Parts  
MunyaComments: I have had my car for 18 months its got 300k kms on the clock been driving well untill some 400kms ago. The fuel gauge became faulty giving false readings, car suddenly lost power on an uphill on one of my drives and I assumed i was running low on fuel. Waited a while and restarted the car it was misfiring, drove the car to the nearest fuel station refueled and the car was okay. The problem became more frequent at times the car would just cut off when idling and required a great deal of patience to start it. When cold it starts well but after 5kms or so it would start misfiring again took it in for diagnostics and did not find any joy.

It seemed like there was a leak somewhere on the intake manifold because after spraying a strong brake disk cleaner the revs would go up but even that test was not consistent. The mechanic replaced the centrifugal oil seperator and there was no change.
I have since discovered that if I keep running it for say a km or two after it starts misfiring the car will clear the misfire when I start it again but this will last about 8 to 14kms before the misfire comes back again.
Out of desperation I took out the catalytic converters because I could pick up a smell like battery acid, assumed it was the cats so took them out. The problem went away for 40km and it started again and the smell is still there.

I need your help what do I do????????????
March 16, 2011
 Followup from the Pelican Staff: Sounds like an issue with the fuel injection system possibly. I would check the trouble codes. It's hard to give any more information - you didn't tell me what car you had. - Wayne at Pelican Parts  
JNolanComments: I have a 1995 BMW 540i that started having a rough idle in cold weather. Then car lost a ton of power. It got to the point that it didnt start at all. Had a new fuel pump and fuse, master cylinder and belts replaced. Car still has check engine light on. OBD code is 1222. We were running it with regular unleaded. Car continued to buck and shudder and would barely go above 10-20 miles/hr. The car almost comes to a crawl on hills. The more gas you give the worse it is. The exhaust smell has a very strong gas smell. Not like rotten eggs but overwhelming smell of gas. Now car will not start at all. Any ideas?
March 5, 2011
 Followup from the Pelican Staff: Code 1222 means that the Oxygen (O2) sensor is detecting a very rich or lean condition. This could be caused by a faulty O2 sensor, or it could be caused by a huge vacuum leak somewhere in the engine. I would perform a check for vacuum leaks in the engine - I have an excellent article on this here: I would also think about replacing the O2 sensor as well - that might help clear up the problem. - Wayne at Pelican Parts  
TomComments: Thanks for posting the article. I have a 2002 330ci with 99,908 miles when the "service engine soon" light came on and the car has a rough idle but still has decent power. I had AutoZone check for codes and they found four. They are P0171, P0174, P1342, and P1348. When I returned home I discovered I had left the oil fill cap off. My guess is that I drove the car between 170-200 miles like that. Thoughts? Oxygen sensor and or mass air flow sensor? Thanks for the help.
February 15, 2011
 Followup from the Pelican Staff: Other than creating a big mess in the engine compartment, there's no real damage that can be done by driving without the oil cap properly attached. I will admit that I did that once too, and paid for it by having to clean the whole engine. The car is expecting there to be crankcase vacuum, so when there isn't, it will pop codes and run rough. Install the oil cap back on if you haven't already, reset the codes, and then see if they return. - Wayne at Pelican Parts  
vinnieComments: Wayne - thanks for this, a vacuum leak is what I have been surmising also. Problem is, where are the vacuum lines? other than the large intake boot that I've already replaced? Can you give me a hintfrom this/to this kind of thing? Thanks much.
December 7, 2010
 Followup from the Pelican Staff: Manifold gaskets can also be sources of leaks - you need to trace everything back from the big boot. I wrote an article on checking for vacuum leaks, you might want to check it out here: - Wayne at Pelican Parts 
VinnieComments: It's a Peake code reader. The codes are:

E4 - 02 sensor adaptation limit cyl 4-6
E3 - 02 sensor adaptation limit cyl 1-3
December 6, 2010
 Followup from the Pelican Staff: Okay, this error can mean that the O2 sensors are reading levels that they should not have been reading (beyond their range). With multiple bank errors like this, it typically means something else is wrong and the fuel injection system cannot adjust properly. The first thing I would look for would be a major vacuum leak - check to see if that is affecting the mixture and metering of the fuel injection. - Wayne at Pelican Parts 
VinnieComments: 2001 330i - I keep getting an E3 and E4 fault code about once a week. I replaced the pre-cat oxygen sensors two months ago, along with a new intake boot both parts, cleaned the air mass sensor, new NGK plugs, new fuel filter, new exhaust camshaft position sensor. Could this be the post-cat oxygen sensors? Also, is there any other vacuum lines to check besides the main large intake boot and flexible tube attached to it?
December 3, 2010
 Followup from the Pelican Staff: E3 and E4, I'm not familiar with what those codes mean, which reader are you using to get them? - Wayne at Pelican Parts 
chefrob15Comments: i own a 1979 mercedes-benz 280 e and i think my oxygen sensor is bad because i am getting poor mpg. i recently had a tune up and the oxygen sensor was not replaced . i have had the car for going on 3 years and have had it tune up twice .could the oxygen sensor be whats causing it to get poor mpg?
October 27, 2010
 Followup from the Pelican Staff: It might be. The fuel injection systems in those days were very primitive in those days, it may or may not be the problem. They are about $30 to replace, I'd say it can't hurt to start there.- Wayne at Pelican Parts 
NishaComments: I have a 1997 BMW 528i. My car has lost a lot of power. I can push the petal to the floor and the RMP's when rise faster then the MPH. The car jerks then speeds up as it changes gears. The cats are not the problem. I already had the taken care of and the problem still exist. I haven't changed the fuel filter yet b/c i don't know if that could cause my car to loss so much power. My car also starts to run hot when i turn on the defrost or the seat warmers. then when i speed up it cools down. I'm thinking i need a raditator fan and a Mass Air Sensor. But I know nothing about cars. Any help? This is drving me crazy.
October 15, 2010
 Followup from the Pelican Staff: You need to read the codes to see what the issues are, see here: The car should give you an indication of what is wrong with it through the DME codes. - Wayne at Pelican Parts 
GeneOComments: I have a 2004 X3 3.0l engine that that runs rough then turns off above 9700 feet. Head back down the mountain and the engine runs great, good fuel economy-power-etc. OB II analysis is a green light and reveals no error code. This is very inconvenient living in Colorado, any ideas. Thanks
October 6, 2010
 Followup from the Pelican Staff: The mixture gets a bit rich at higher altitudes due to the less-dense air. All modern fuel injection systems have some method to determine the density of the air. I'm not specifically sure about the 2004 model - it may be built-in to the mass air flow sensor (MAF). I would check, and if there is no specific altitude (air pressure sensor or barometer), then I would think about replacing the MAF as a potential solution to the problem. - Wayne at Pelican Parts 
beastComments: My daughters 2001,330i started suddenly running rough. Its real bad at idle. I pulled the codes. Three were for misfiring cylinders and one for a,"slow response oxygen sensor bank 2 sensor 2." I just had the A/C serviced and she said she almost ran out of gas right before it started running rough. Could the fuel filter be plugged?
July 8, 2010
 Followup from the Pelican Staff: If the O2 sensor is giving a fault code like that, I would reset the codes and see if it returns. - Wayne at Pelican Parts 
cooper sComments: i have a 08 mini cooper s and it misfires only for 10-15 seconds after a cold start and seems to be overheating slightly. could this be a o2 sensor?
June 22, 2010
 Followup from the Pelican Staff: It could be. I would get a Peake Research reader and pull the codes. I don't have a MINI specific article up just yet, but this one is very similar: - Wayne at Pelican Parts 
actuatorComments: actuator's solution. Found that my o2 sensor heat relay was not quite seated all the way. Noticed the relay next to it was the same part number too. Swapped relays and check engine light went away immediately after starting car. Thank you for your article on o2 sensor troubleshooting. It was very informative.
June 5, 2010
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
actuatorComments: I have a 1996 bmw 318ti. My service engine light came on and I went to Autozone to have them read my codes. They showed me that my car had a "faulty HO2S11 bank 1 sensor 1" In other words a faulty upstream of the catalytic converter 02 sensor. they said my car would use the Bosch p/n 13559 sensor. installed sensor and light was still on. They reset the light and it came on again 20 miles later. Is the 13559 the correct part? I cleaned the old one and it worked for a while about 10 miles. Any suggestions? Thank you in advance. actuator
June 3, 2010
 Followup from the Pelican Staff: Two suggestions - it may be the wire harness is having trouble, or you may have replaced the wrong bank? - Wayne at Pelican Parts 
JDComments: I have a 2005 Nissan Altima 2.5L. Just recently the engine light came on. When I try to start the car it doesn't start right away. It seems like it's not getting gas. The car will start after a few minutes of trying. Would this be caused by the O2 sensors or the catalytic converter going bad?
May 25, 2010
 Followup from the Pelican Staff: It could be caused by just about anything. Pull the trouble codes and they will tell you most likely where the problem lies. - Wayne at Pelican Parts 
LouComments: I've recently had mechanic drop the pan, drain the transmission fluid in which 75% was changed, they checked magnets and found some slushy build up but not excessive, and they changed the screen. This is as much of a tranny service they recommend.
May 25, 2010
 Followup from the Pelican Staff: I you are not having problems, that is the only maintenance there is.
- Nick at Pelican Parts
LouComments: Wayne,
Thanks for the response. I've had three transmissions mechanics test drive it and none of them think it is a transmission problem, of course none actually felt the symptoms of power loss and high rpms during the test drives. I've taken the Infiniti to the BEST transmissions shop in my city of 1.7 million residents, and he is now refering me to an electrical specialist to test the speed sensors and more. Oh, I forgot to mention that codes being given are for SPEED SENSOR not sure which one and CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD. From what I have read, many others with the same symptoms as my car have solved the problem by replacing the necessary speed sensors. I'm really of skeptical of any mechanic who insists on overhauling the transmission without even doing a minimum diagnostic, which in my book should involve testing the speed sensors. Any thought on this? Thanks
May 25, 2010
 Followup from the Pelican Staff: "Of course none actually felt the symptoms of power loss and high rpms during the test drives" Right, so they haven't seen the problem. I would change the fluid in the transmission and cross my fingers. - Wayne at Pelican Parts 
LouComments: I have an Infiniti i30 2001 with 120,000 miles. Problem is: Almost daily the car will lose power while going some 20 - 40 mph and rpms will shoot up to almost 4000. It feels as if transmission were slipping. Each time this happens I have to pull over, shut off the engine and turn it back on to continue with my trip. I've taken it to several mechanics. All have ruled out any transmission problems. When they do a computer scan, codes for catalytic convertor pop up. One mechanic says that Nissan says to not touch catalytic, instead to change the o2 sensors. Well, i've changed both rear o2s and now this mechanic says both front ones should be changed. Any advice? I realize I am over the factory 100,000 suggested miles, at 120k, but could the o2s really be causing this? Oh yeah, also engine is running rich.
May 22, 2010
 Followup from the Pelican Staff: This sounds like an intermittent transmission problem with nothing to do with the O2 sensors. I would focus further attention on the transmission. - Wayne at Pelican Parts 
TEEMComments: I have a 2007 Audi A4 3.2L. It sometimes hard starts turns over for 10-15 seconds before it fires, but runs and idles fine, though the exhaust smells rich, runs hot and blows out some black smoke when it's cold. When I am accelerating at between 1500 and 2500 RPM in the higher gears, I get a random misfire. I have recently replaced the plugs and coil packs, as Audi has a Maintenance Campaign out for the packs, and it was diagnosed today as having a bad o2 sensor. Could it be just a sensor? Or could it be something more? Thanks.
April 29, 2010
 Followup from the Pelican Staff: The O2 sensor could cause performance issues, but I don't think it would cause the types you're referring to here. I would get an OBD-II code reader and see what other codes are being tripped by the computer. - Wayne at Pelican Parts 
WaltComments: I have four o2 sensors. two in front of cat. converter and two behind. Dual exhaust. After engine warms up it runs rich and idles rough. When I accelerate it blows out a cloud of black smoke.I've replaced all the o2 sensors,but not all at the same time. any suggestions?
April 18, 2010
 Followup from the Pelican Staff: Check your fuel pressure regulator for leaks, Check the fuel pressure, check the injectors for leaking or sticking open. Also look for a vacuum leak.
- Nick at Pelican Parts
JeffComments: Canuck Van, all dodgesmaybe other brands too? have a built in diagnostic code reading facility.. Early models like my '94 b250 van show the codes as a series of blinks through the engine light, and newer ones with digital dashboards show them as actual codes in the odometer.. google dodge trouble codes or something till you find the procedure.. it works, I've confirmed it.
April 5, 2010
 Followup from the Pelican Staff: Thanks for the additional Info. We appreciate it.
- Nick at Pelican Parts
kelsComments: My service engine soon light came on. I took it to a shop and the guy there said it was the oxygen sensor and that sometimes they just get dirty or moisture builds up on them and they misfire or misread. He turned off the light but didn't replace the sensor. He said to drive it a while and if the light comes back on then to bring it back. Will this do anything to my car? I have a trip coming up where I'm going to be driving almost 200 miles, will this be bad for my car?
April 1, 2010
 Followup from the Pelican Staff: A faulty o2 sensor will cause the engine to use more gas, it won't really do any damage to the engine.
- Nick at Pelican Parts
BOComments: 1988 325ic check engine light is on , fault code found OA oxygen sensor faulty? thanks
March 27, 2010
 Followup from the Pelican Staff: The o code is for an insufficient ground connection between the DME and the sensors, the A code seems to be related to the air bag system.
- Nick at Pelican Parts
pattyotoolComments: My check engine light is on, I believe my oxygen sensors may be bad. However the engine also surges at half speed and gets worse when I open the throttle. I replaced the plugs and the fuel filter which was bad. Not sure if I change the sensors that will repair the problem.
February 22, 2010
 Followup from the Pelican Staff: What year and model car do you have?
- Nick at Pelican Parts
stringerComments: My car has 2 O2 sensors before the cat and 2 O2 sensors after the cat. It's a '99 BMW M3 One of my pre-cat sensors indicated bad from the scan tool. Do I have to replace both pre-cat sensors, or just the bad one? The car runs fine, and only has 45,000 miles.
February 22, 2010
 Followup from the Pelican Staff: You can just replace the one sensor that has set the fault.
- Nick at Pelican Parts
RaymondComments: Vey good article, alot of great info for trouble shooting.
February 12, 2010
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
Canuck vanComments: Thanks for the article. After new plugs, wires, and air filter, my 1990 Caravan 3.3L starts fine, idles fine, and revs fine in neutral BUT as soon as I put it in gear and accelerate even slowly, it starts misfiring after about 1600 rpm, and it craps out unless I ease off the pedal...I've pulled the 02 sensor 4 wire and the shield vents aren't blackened or plugged...I don't have a digital voltmeter. Do you think its the 02 sensor?
December 10, 2009
 Followup from the Pelican Staff: Your car will output trouble codes - you need to figure out what codes are being triggered, otherwise it's just guesswork. The Durametric tool we sell should help diagnose this, or you can take it to a good independent shop that has an applicable tool. - Wayne at Pelican Parts 
oninComments: hi there.. i have a 2003 honda city i-dsi with the same symptoms stated above. i already had a check run to the computer system and it was pointing to the o2 sensor of code 2 i it the o2 sensor already? thanks a lot
October 20, 2009
 Followup from the Pelican Staff: Yup, sounds like it's the 02 sensor. - Wayne at Pelican Parts 
CurtisComments: Excellent article. I appreciate the tips and symptoms, especially.
May 21, 2009
 Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts

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