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Oxygen Sensor Troubleshooting and Replacement

Pelican Technical Article:

Oxygen Sensor Troubleshooting and Replacement


1/2 hr


$50 to $150





Applicable Models:

Porsche 911 (1980-89)
Porsche 930 Turbo (1986-89)

Parts Required:

New oxygen sensor

Hot Tip:

The grommet that goes in the engine sheet metal comes attached to new OEM sensors

Performance Gain:

Better air/fuel mixture and better engine performance

Complementary Modification:

Catalytic converter replacement
101 Projects for Your Porsche 911

This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.

The modern fuel injection systems utilize a form of closed-loop feedback to regulate the air/fuel mixture within a specific range. Beginning in 1980, Porsche installed an oxygen sensor (sometimes called a Lambda sensor) to better regulate the mixture of air and fuel being injected into the CIS fuel system. Not only does this sensor enable the catalytic converter to work at its maximum efficiency, but it also helps the car run more efficiently with more power.

The oxygen sensor is one of the most important elements of the modern fuel injection systems. Early fuel systems that utilize carburetors can see the air/fuel mixture ratios vary as much as 20% over the operation range of the engine. In comparison, a finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02%. Keeping the engine at the stoichiometric level (14.6:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.

The oxygen sensor is located in the exhaust system of the engine, and senses the oxygen content of the exhaust gases. 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.

The oxygen sensor is manufactured out of a ceramic material called Zinconium dioxide, and the inner and outer surfaces of this ceramic are coated with platinum. The inner surface of the sensor is exposed to outside air, while the inside surfaces are exposed to the exhaust gases. A small voltage signal is produced due to the difference between oxygen contacting the inner and outer surfaces.

If the amount of oxygen in the system is low, then the sensor will produce a high voltage signal (around 900 milivolts) and the ECU will compensate for the mixture being too rich. The amount of fuel delivered to the engine will decrease.

If the amount of oxygen is too high, then the oxygen sensor will send a low voltage signal (around 100 milivolts) indicating a lean mixture, and the ECU will adjust by adding more fuel to the injection system.

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

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. If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running really lean, 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 1.5V, 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 the scope of this project. If you think that the oxygen sensor may be causing some of your fuel injection problems, it should be replaced. In general, it is recommended that you replace the sensor every 30,000 miles. The sensor is located on the left side of the car, right before the catalytic converter. Access is made much easier by removing the rear left wheel. The sensor is simply unscrewed from the converter. If there is a lot of rust in this area, it may be advisable to spray the area with some lubricant and let it sit overnight prior to attempting to remove the sensor. When you install the new one, it's a wise idea to place some anti-seize compound on the threads so that they will not rust and seize up. Some of the new sensors even have the anti-seize already placed on the threads right out of the package. It's also important to note that the rubber grommet that is attached to the wire on the sensor usually comes preinstalled with each new original equipment sensor.

The oxygen sensor (O2 Sensor) is attached to the top of the catalytic converter.
Figure 1

The oxygen sensor (O2 Sensor) is attached to the top of the catalytic converter. The sensor measures the amount of oxygen in the exhaust prior to the catalytic converter acting on the exhaust. The sensor is one of the key elements in a 'closed-loop' feedback system that regulates the amount of fuel injected into the engine. The test port, located directly below, allows the measurement of exhaust gases to be made before they enter the converter. The catalytic converter pictured is an aftermarket unit, and not an original Porsche catalytic converter.

Comments and Suggestions:
Shawn Comments: Thank you for the reply regarding the o2 reset sensor button. Since I pushed the button on the small black box and the light did not go off ...any suggestions? I have looked closely with a flash light and can see no more buttons to push . I have replaced the o2sensor so that is good. When is it time to just pull the bulb? Thanks again!
November 4, 2016
  Followup from the Pelican Staff: Could be the timer in the gauge is faulty. You can remove the bulb. - Nick at Pelican Parts  
Shawn Comments: After pushing in the spring loaded button behind the speedometer the light never did go off . Is the button suppose to stay depressed when reset or does it come back to the position I started with?

November 3, 2016
  Followup from the Pelican Staff: it should not stay down, pressing it performs the reset.

This box counts the mileage and after 30,000 miles turns on the light. Hit the button and the light should go out. - Nick at Pelican Parts
Alberto Comments: Greetings from Argentina.
I brought my '79 928 here from Canada, it has been running very good until about a couple of months ago when the car would start, run for a few minutes and then stops suddenly.
I thought then that it was out of fuel gauge is not working..I filled the tank...and the same thing happened.
I checked the fuse it was OK but the relay was hot kind of warm is a better description.., I took it out cleaned and filed it a it...but NO change.
Would a bad relay be the problem?..., is there a way of checking if it is bad?.., there is none around here for sale, so the intention would be to check if it is the relay the culprit.
Many thanks,
March 17, 2016
  Followup from the Pelican Staff: Yes, first confirm the pump isn't getting good power and ground.

When your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector spray and fuel pressure, volume, quality and engine compression. Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
Dgolia Comments: I have a 1983 911. It is the European model which means there is no catalytic converter. Is there still an oxygen sensor? If so, where is it located?
August 22, 2015
  Followup from the Pelican Staff: You should have one, it wold be in the exhaust, next to the cylinder head. - Nick at Pelican Parts  
A. Miguel Comments: I sent my '79 928 to SouthAmerica for "future enjoyment". Before the car was shipped, the local dealer here in Canada suggested that "at least" one of the cat converters will need to be changed soon. Since in m country there are no emission regulations for a car this old, I was thinking in removing ALL the converters and replacing them with a sport custom madeexhaust, would this change help the car's performance?...what about the oxigen sensor?
Your opinion will be appreciated.
Many thanks.
February 26, 2015
  Followup from the Pelican Staff: A professionally engineered exhaust without catalytic converters will increase performance. You may find a decrease in throttle response at lower RPM, but you'll pick up some power at higher RPM. As for the oxygen sensor, I would keep the sensor in the loop. Reason being when you are cruising the sensor will be mindful of the air fuel ratio saving fuel, when you mash the throttle to wide open, the DME (engine control module) ignores the oxygen sensor and the fuel mixture goes to a rich mixture which Porsche engineers have determined works better for producing power. So removing the sensor will not increase performance. Remember that you'll be driving a car with no catalytic converters, should you choose to remove them, and the exhaust fumes will be dramatically increased. - Casey at Pelican Parts  
Ralph Comments: I have an '87 944 n/a and I have been chasing a stumbling acceleration for a year or more. I have swapped the AFM and DME with known good ones, replaced the TPS, ignition system totally, vacuum lines all, Catalytic Converter, intake manifold gaskets, fuel injectors, re-worked the wiper on the AFM and a new Bosch O2 sensor. Nothing worked until I disconnected the O2 sensor. Now the car runs great except the idle cycles 200 rpm. I have no idea what my mixture is now rich vs lean but I think I'm onto something.
July 21, 2014
  Followup from the Pelican Staff: I don't feel you have any real direction toward the cause of the issue. Start by confirming fuel pressure, volume and quality are OK. Then check the engine for vacuum leaks. You need to confirm a few things are working well before digging too deep. This could be anything from an engine mechanical issue to a shirted o2 sensor. - Nick at Pelican Parts  
cavazosnacho Comments: I bought a porsche 924 1978 and has headers, but no sensor in the exhaust system, as I understood, the oxygen sensor began in 1980, my question is if my model needs oxygen sensor or some other type on muffler, Thank you.
April 11, 2014
  Followup from the Pelican Staff: Your vehicle may have been equipped with an oxygen sensor, as one is available for it. Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.
- Nick at Pelican Parts
car man Comments: The question about the 02 sensor connector is for a 1984 Porsche 911, thanks again
March 3, 2014
  Followup from the Pelican Staff: Thanks, got it. - Nick at Pelican Parts  
car man Comments: This question for Bob Tindel,your parts dept. advised me ask you. Replace O2 sensor, connector end coming out of main wiring harness broke off. it's green wire with a small wire in middle and another wire wrapped around it. can the two wires be wired together to put a connector back on. Thanks
March 3, 2014
  Followup from the Pelican Staff: Yes, you can repair the wiring. Be sure to use butt connectors and heat shrink tubing to seal the connection. I would be sure to wrap the wire with the shield once you but connect it. Make sure the shield (sheilded wire) does not come in contact with the sensor wire. - Nick at Pelican Parts  
car man Comments: while replacing the Oxygen sensor the plug end going into the body broke offnot the end on the sensor. what do I do now ? is there a replacement part for this problem. Thank you
March 1, 2014
  Followup from the Pelican Staff: The connector should be available. Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.
- Nick at Pelican Parts
newk00 Comments: Hi, I am ready to replace the oxygen sensing unit, however, the part that is broken is located just below the fuel filter. It is an orange plastic piece male and the black female end from the sensor unit goes over top of it like a spark plug cap. What is the orange unit called and can I get it at Pelican parts? It seems to be connected to a green wire which might run into the cockpit. Thanks for any help, John
February 8, 2014
  Followup from the Pelican Staff: I'd have to see the part to be able to identify it. Give our parts specialists a call at 1-888-280-7799. They can help also. - Nick at Pelican Parts  
rinverso Comments: where does the other end, the plug go in the engine compartment? the existing one is not connected, so i don't know where it goes.
April 17, 2013
  Followup from the Pelican Staff: The oxygen sensor connector is located on the left or right side of the engine compartment. It has a square connector with a slding lock. - Nick at Pelican Parts  
Jeanne Comments: Thank you.
November 2, 2012
  Followup from the Pelican Staff: Remove your speedometer and on the cable you will see a box with a white button. Push the button and the O2 sensor warning light will go out. It will return again after 30,000 miles have clocked on the odometer. - Nick at Pelican Parts  
Mickey Mouse Comments: when replacing oxygen sensor is there a switch to reset so that dash warning light goes out?
September 28, 2012
  Followup from the Pelican Staff: Yes remove the speedometer and you will see a box with a white button on it. This box counts the mileage and after 30,000 miles turns on the light. Hit the button and the light should go out. - Nick at Pelican Parts  

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