The oxygen sensors (also called O2 sensors) are one of the most important elements of the modern fuel injection systems. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric ratio (14.7:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensors are located in the exhaust system of the engine, and they sense the oxygen content of the exhaust gases. There are two on this Mercedes Benz M104 engine: one on each side of the catalytic converter. The sensor located just in front of the catalytic converter measures the mixture of the exhaust gas exiting the engine. The sensor located after the catalytic converter is used to measure the performance of the converter by comparing the O2 levels before and after. The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.
There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are:
Irregular idle during warm-up
Irregular idle with warm engine
Engine will not accelerate and backfires
Poor engine performance
Fuel consumption is high
Driving performance is weak
CO concentration at idle is too high or too low
Check Engine Lamp is illuminated
In general, if the oxygen sensor is not working, the car will be running very poorly, and will also be outputting a lot of harmful emissions. The cars computer will usually give a warning signal that lights up the Check Engine Lamp if the signal received by the computer is out of its normal range. Sometimes the computer may output an error code stating that the oxygen sensor is reading out of range, when in reality the values registered by the O2 sensor are accurate because there is something else wrong with the fuel injection system. Prior to replacing the oxygen sensors, make sure there are no other codes being recorded that may affect the O2 sensor readings..
If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running lean (not enough fuel), and will try to richen the mixture. At the other extreme, if you disconnect the oxygen sensor, and replace it with a small AA battery that supplies 1.5 volts, the ECU will think that the car is running really rich and attempt to adjust the mixture to be leaner.
Needless to say, troubleshooting the complete fuel injection system is beyond this project's scope. If you think that the oxygen sensors may be causing some of your fuel injection problems, they should be replaced. In general, I recommend that you do this as needed. You will have to jack up the car and secure it safely on jackstands to gain access to the sensors.
Using a 22mm wrench, simply remove the rear sensor from the exhaust pipe. On the front oxygen sensor, you’ll need to get a little creative. The front sensor is mounted in a really tight space and at a odd angle. To loosen it, you’ll need to use a special type of crow’s-foot wrench with a slit cut in the side to remove it. The electrical plug for both O2 sensors unplug from the chassis plug. New O2 sensors should have the same exact plug - ready to attach to your car. When you remove the O2 sensor, you will probably find that it is coated with black soot. This is normal for an old, worn out O2 sensor and usually indicates a rich running condition.
Install your new sensor snug-tight, or if you have the proper slit-tool and a handy torque-wrench, then tighten it to 40 ft-lbs (55 N-m). It's also a smart idea to add some anti-seize compound to the threads of the plug before you install it, but make sure the anti-seize doesn't get into any of the slits on the head of the sensor. Check the sensor first though, as new ones usually come with a dab of anti-seize already on the threads.
There are two different types of sensors you can purchase: generic ones that allow you to snip the connector off of the old sensor and put it on the new one, and original OEM sensors with the correct connector. On older cars, I used to use the generic sensors, but I've had problems with using them on these newer cars. Researching further, I discovered that the wires and connectors are very important on these O2 sensors.
Soldering wires together can interrupt the reference air signal and lead to problems with the sensor. I've also had problems with the Bosch factory Posi-Lock connectors that are supposed to work with these newer sensors. Because the relationship between the connector and the O2 sensor is so vital to the proper reference signal, I recommend that you only use the correct OEM sensors with the proper plug.
Jack the car up and secure it on jackstands at all four factory jacking points. You’ll need to get under the center of the car to gain access. Shown here is the rear oxygen sensor in place for a Mercedes-Benz with the M104 engine (green arrow).
Use a 22mm wrench to loosen and remove the rear oxygen sensor. Once removed, pull the cable free of the clips that hold it in place and unplug it from the electrical connector. Place the new sensor in the exhaust, torque it to 40 ft-lbs (55 N-m) and plug the connection in after routing the new cable along the clips holding it to the car.
Shown here is the top of the front mounted oxygen sensor (green arrow). As you can see, it’s a tight fit in there. What makes it even more difficult is the angle at which the sensor is mounted. It faces into the exhaust stream right at the merged collector between both header banks. This angle will prevent you from fitting a wrench over the sensor.
Slide the crow’s-foot wrench over the sensor and use a ratchet with extension as shown here. It’s tricky to get it at just the right angle that will allow you to loosen the sensor but it is possible. NOTE: if your transmission mount has failed, there will be almost no way to get the wrench on the sensor. Just something to keep in mind if it seems like you just can’t reach the sensor. See our article on transmission mount replacement for more info. On our project car, the transmission mount had failed, causing the whole transmission to sit about 2 inches lower. Once the mount was replaced, it became evident how to get the crow’s foot wrench over the sensor.
Here’s a shot of the oxygen sensor being removed from the side. As you can see here, you’ll need to reach the ratchet over the exhaust pipe and then fit the extension into the crow’s-foot wrench. Once removed, take the new sensor and carefully thread it into the exhaust. Torque down the sensor to 40 ft-lbs (55 N-m) and plug the connection in after routing the new cable along the clips holding it to the car.
Comments: I have a 1996 SL320 that recently had new spark plugs put in. Lately, my mileage has gone way down 13 MPG city from 17-18. Also I feel the engine seems slightly rough at idle. Would this possibly be the oxygen sensors car has 92,000 miles on it
March 2, 2014
Comments: I need an o2 sensor connector body side and can't find one. 93 400E. Your article above sounds as though it crosses with several other vehicles. Can you help? No donor cars at salvage yards near me.
February 25, 2014
Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799. They can help you find the right part. - Nick at Pelican Parts
Comments: I am looking for the O2 heater relay module on a 1994 SL320 in the component locators shows it next to the computer but it is not there. can you tell me where to find it
February 18, 2014
Followup from the Pelican Staff: I don't see that component in my locator. Does the wiring for your vehicle show that it exists? - Nick at Pelican Parts
Comments: The reglutor number is bosch 0280101008
February 18, 2014
Followup from the Pelican Staff: Thanks, got it. - Nick at Pelican Parts
Comments: I have a c36 amg 95 . I check if it have o2 sensor . There was not o2 sensor . But behind the ecu there was a reglator sensor . Some help please .
February 18, 2014
Followup from the Pelican Staff: I believe that is an ambient pressure sensor. - Nick at Pelican Parts
Comments: Thanks Pelican staff the codes are P0132,P0130, P0171.This was what the mechanic gave me.
February 3, 2014
Followup from the Pelican Staff: These are oxygen sensor and lean codes. I would the fuel trim and see what it is doing. Also check for vacuum leaks and operation of the mass air flow sensor. - Nick at Pelican Parts
Comments: 1 have a 2004 C180 compressor coupe the engine light is on.What type os sensors do i need to get?
January 29, 2014
Followup from the Pelican Staff: No idea. Start by reading the fault codes. - Nick at Pelican Parts
Comments: hi I have a 2003 kompressor a code came up for a altitude sensor is that the same as an o2 sensor or something else.
January 19, 2014
Followup from the Pelican Staff: No, the altitude sensor would be the map sensor. - Nick at Pelican Parts
Comments: dear Sir: I have a w140 S320. I could not use a crows foot wrench to remove oxygen sensor.Just no room to put a rachet on.I used a Snap On 22mm oxygen sensor tool,it has a six sided socket and a handle.
January 14, 2014
Followup from the Pelican Staff: Thanks for the additional info. We appreciate it. - Nick at Pelican Parts
Comments: I found the post here very informative. Car Guy's match 26th comment caught my eye as I have a 2005 SL500 in which the engine light pointed to the O2 senors. I was told there was four. That started me looking for the actual senors. I see a front, upstream, rear, before Cat and after Cat that's 5 unless it a matter of wording. Also does every senors have to be replaced. Is it possible to narrow it down to the actual culprit.
December 21, 2013
Followup from the Pelican Staff: The fault code will direct you to which sensor it is. For example B1S1 would the sensor before the cat on bank 1, bank 1 is where cylinder 1 is. - Nick at Pelican Parts
Comments: Is Bosch Universal Oxygen Sensor Installation Video Correct?
It is very hard to believe the electrical wires can let air go through: http://youtu.be/3Yt6Lg0oU58
December 2, 2013
Followup from the Pelican Staff: This has been discussed many times before here at Pelican HQ. I've written five books on Porsches and BMWs, and the fact of the matter is that I still don't know for sure. The goofy posi-lock system that BOSCH has for these connectors seems a bit odd to me. In general, the O2 sensors are supposed to be water-proof, meaning that they have to work submerged in water. So, the air-reference signal that the sensor needs to measure O2 has to be gathered through the connector itself. I'm not sure if soldering by itself is the issue with not having a proper connection, or whether it has to do with the fact that people seal the connection with tape and/or heat-shrink tubing. I would agree that soldering by itself would probably not cause a problem. The act of covering up the soldered connection is what causes issues, I believe. This is just my opinion - I'm not 100% sure. In all of the books that I've written, I recommend that our customers stay away from these generic sensors and instead use the proper OEM BOSCH ones with the proper car-model-specific connector installed and attached at the BOSCH factory. - Wayne at Pelican Parts
Comments: Your Website is wonderful. I'm learning a lot.
The MB E240 automatic: how many O2 sensors. Because on the exhaust I see 1 2 of each side before the catalytic and 2 as well after the catalytic. I don't know if I'm correct by saying 4?
November 25, 2013
Followup from the Pelican Staff: There are four total. - Nick at Pelican Parts
Comments: hi my car is E240 2003 after receiving engine heat 80 degree centigrade .
Fan start and then move very fast. And it hasn't decreasing of speed . why ?
November 23, 2013
Followup from the Pelican Staff: Keep in mind this car was never sold in the USA where we are. It sounds like it might be a bad CTS or connector. When the coolant temperature sensor is unplugged the fans will run all the time. - Kerry at Pelican Parts
Comments: hi my friend
I have mersedes e 240 2003 .
what can I clean ecu computer code?
can you help me?
November 23, 2013
Followup from the Pelican Staff: I don't know where you are but you can usually go to an auto parts store chain to pull and clear codes. You may be able to disconnect a battery for a few hours but adaptations will be lost and the anti-theft system may prevent the car from starting when you reconnect the battey and need a scan tool relearn procedure. The safeest way is with an OBD-II compliant scan tool. - Kerry at Pelican Parts
Comments: i just got a 1994 400 sel i was driving it the other day a loss power it wounld start but when i put it in drive it cut out 20min later i try it ran and stop 20min same thing is it may fuel pump
October 19, 2013
Followup from the Pelican Staff: Yes you may have a bad fuel pump. Try removeing the gas cap. You may be developing a vacuum in the tank because the vent in the casiter is clogged. - Kerry at Pelican Parts
Comments: recently changed all air intake boot / hoses on my S500 W140 Benz 1994 model due to hoses cracked by age. now, suddenly my vehicle is emitting lot of white smoke and irregular idle while warming up. coolant is OK and also engine oil is in between max and min level. what is the cause, Please.
October 15, 2013
Followup from the Pelican Staff: White smoke is either brake fluid, coolant or water in the gas. Try another tank of fuel (petrol) and see if this does not help your problem. - Kerry at Pelican Parts
Comments: E320 CDI. When key was turned on, pre ignition heater lite pigtail came on and stayed on about 30 seconds before going out. Check engine lite came on and stayed on. Engine runs great and temperature stayed normal. I am about 300 miles from home. Please advise if it is safe to travel that distance.
October 12, 2013
Followup from the Pelican Staff: It is impossible to say without knowing what the trouble code is but if the engine and transmission are working normally you're probably okay - Kerry at Pelican Parts
Comments: I have a 1994 E320 showing trouble code 2 DTC Memory. It states, "Heated oxygen sensor inoperative." Since there are two O2 sensors, according to your article, which one does this trouble code refer to?
September 24, 2013
Followup from the Pelican Staff: Your vehicle is a 1994 and thefore not subject to OBD II regulations, You only have one O2 sensor before the catalytic converter in the exhaust manifold or the downpipe. - Kerry at Pelican Parts
Comments: Hey Fupabox, im a fan of your youtube page with all your DIYs but I have a '94 w202 that has a rough idle. With this it also hesitates on acceleration, the rpms fluxuate a lot, and an occasional smell with a rare sight of white smoke. The car has 254,000 miles but i just had a full 400$ tune up on it. Would a dirty throttle body be the cause of this? Idk if i should try this yet.
August 12, 2013
Followup from the Pelican Staff: Since I don't know if you have the 104 6 cylinder engine or the 111 4 cylinder engine since you gave me the chassis and not the model it's hard for me to tell. White smoke is usually coolant so you could have a bad head gasket or you could just have water in the fuel. You could also have a vacuum leak causing your idling problems. Do you have an American model with EA (Electronic Accelerator)? You could have the problem with your EA module. - Kerry at Pelican Parts
Comments: Hello I have a light on in my c220 code says mass air flow sensor I replaced them both 2 years ago . Can they go bad that fast?
August 2, 2013
Followup from the Pelican Staff: ? Do you have a twin throttle body? There is usually only one Mass Airflow sensor. Do you mean oxygen sensor? Both MAFs can get dirty and oily and can be cleaned. O2 sensors can go bad if the vehicle is driven with misfires and leaky head gaskets contaminating the O2 sensors with coolant leaking into the combustion chamber. - Kerry at Pelican Parts
Comments: Followup from the Pelican Staff: Hello Anumohan IMO: 90% odds the EGR valve or passages are plugged = clean or replace as needed.
Here is a list of the possible causes.
Possible causes * EGR valve or passages plugged = clean or replace as needed * Sensor damage * Wire harness damage * Update control unit software * Powertrain or ECM failure * Catalytic converter damage
Thanks for the valuable comments,
But still can you assist me with the following
Possible causes * EGR valve or passages plugged = clean or replace as needed Is this something I can do myself, If yes please can you post some pics of EGR valve and passages
* Sensor damage - Its EGR sensor right?
July 29, 2013
Followup from the Pelican Staff: Sorry, we don't have a DIY on this.
It is a moderately high level procedure that most shops can have done in one day.
I can't speculate further without hands-on diagnosis.
I have a Mercedes W210 2001 Model E240. Currently im facing poor fuel efficiency, Im getting 350 to 400 Km for a full tank in short runs and 450km on pure highways. My previous owner says he was getting 450 for short runs and 500+ on highways. i Changed air filter, 12 sparkplugs, AirMass sensor, but still no progress. the scan says Code P2001 Malfunction of Exhaust gas recirculation Functional Chain P0400. Is this something to do with O2 sensor? Can somebody suggest me something to improve the fuel efficiency.
July 27, 2013
Followup from the Pelican Staff: Hello Anumohan
IMO: 90% odds the EGR valve or passages are plugged = clean or replace as needed.
Here is a list of the possible causes.
* EGR valve or passages plugged = clean or replace as needed
* Sensor damage
* Wire harness damage
* Update control unit software
* Powertrain or ECM failure
* Catalytic converter damage
Comments: I want photo of a oxygen sensor on a Mercedes Benz
July 18, 2013
Followup from the Pelican Staff: I assume you are installing a factory replacement oxygen sensor, and can't find the connection.
Look under the passenger front seat, it may be easier to remove the seat for open access of the connection.
Comments: What are the symptoms of a faulty engine temperature sensor?
June 5, 2013
Followup from the Pelican Staff: Sorry, I can't answer this question.
where is the Evaporative System Pressure Sensor located on 1998 c230 mercedes benz?
June 4, 2013
Followup from the Pelican Staff: Under the body behind a panel with the charcoal canister, and up by the engine.
Comments: I have a Mercedes 1998 C230...a code of P0450 which referred to emission and pressure sensor...is this the O2 sensor going bad? engine is on... where are they located at?
June 2, 2013
Followup from the Pelican Staff: Sorry, this has nothing to do with the O2 sensor.
P0450 is Evaporative System Pressure Sensor Malfunction.
In simple terms, you have a damaged sensor that set this code.
Comments: i had local help from a mechanic specialized in German cars.And he"s the one who has led me to make all the previously cited changes but the problem persist.These MB-Codes P1444 and P0131 are the ones that diagnostic tool have raised.Please just give me ur expert.opinion...Could you please Elaborate on the bank1 & bank2...rear O2 sensor and where they are located...thx
May 29, 2013
Followup from the Pelican Staff: Sorry, this requires hands on diagnosis and testing.
My best advice: Take it to your local Mercedes Benz dealer for correct diagnosis and repair.
Comments: Can you explain a bit for bank 1?Does it mean there is another rear O2 sensor for bank 2?Where is it located? I was also diagnosed MB-Code P1444 Regeneration switchover valve. Is it the part in the pic attached? My car is having strange behavior when cold loss of power, rough idle, no CEL though but when warming it for like 20min or just leaving it to warm on the sun it runs perfectly right...I've already changed the air mass meter, air filter, spark plugs, engine wiring harness all with no effect...Could these two codes P0131 and P1444 be the reason? am in despair please help...
May 28, 2013
Followup from the Pelican Staff: Sorry.
You need hands on (local) diagnostic help. - whunter
Comments: I've been told there are the front and rear O2 sensor.which one could be responsible of raising MB-Code P0131 O2sensor voltage jump too slight
May 28, 2013
Followup from the Pelican Staff: This code is for the rear o2 sensor for bank 1. This code means the sensor is reading a lean condition. An exhaust leak may cause the same code. - Nick at Pelican Parts
Comments: How many O2 sensor does a C220 W202 1994 petrol got? And where are they located?
May 28, 2013
Followup from the Pelican Staff: It should have only one mounted in the exhaust manifold mounted near the back of the engine - Nick at Pelican Parts
Comments: on a r129 300sl-24 with a m104 engine does it only have one oxygen sensor or two
May 11, 2013
Followup from the Pelican Staff: One sensor, glad you found the diy useful.
Comments: Thanks for the info Roy.
A friend managed to get the All Data wiring diagram for my car and it shows the following wiring connections:
Bosch Sensor Black signal wire to the Green thick car.
the White heater wires go to Blu/Grn and Brn and the Gry to Blk. Note the Pelican Parts Universal Bosch sensor wires were too short so had to extend them and use heat-shrink sleaving.
Have not yet been on a long run to check the codes yet.
April 12, 2013
Followup from the Pelican Staff: You are welcome.
Comments: I have a 94 E320 with the 104 engine. The O2 sensor needs replacing so I bought the generic one. I cut the old wiring under the passenger carpet so I could remove the sensor but did not mark the wiring connections dumb!!!. There is a thick green wire with a single connector which I assume is the signal wire and connects to the black from the O2 sensor. The other car harness wires are black/green, black and a brown wire. Which wires do the two white heater wires and grey wire from the O2 sensor connect to?
April 2, 2013
Followup from the Pelican Staff: This is the best data I can find.
Comments: I have a 2004 mercedes SL55 AMG an the repair person said i have 4 co2 sensors on my car, but it only shows 2 sensor before an after which is it 2 or 4 sensor
March 26, 2013
Followup from the Pelican Staff: Mercedes EPC says one before and two after the catalyst.
After market sources say two before and one after the catalyst.
Both show a total of three.
Comments: I have a Mercedes 1995 E320 Cabriolet. While looking for oxygen sensors I have found many available for "upstream" but nothing for "downstream". One blog says there is only one but I can't double check that. Anyone know?
March 25, 2013
Followup from the Pelican Staff: Mercedes EPC shows only one (upstream) O2 sensor for your car.
Comments: 1996 E320 per the error code, changed Mass Air Flow Sensor but code still there. Planned to change 02 sensor as next step. The shop checked my car and said there is only ONE 02 sensor. Is that possible or he may just didn't find the other one??
November 7, 2012
Followup from the Pelican Staff: There is only one O2 sensor on your vehicle. - whunter
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