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Everything you need to know about Oxygen Sensor!

2nd July 2007

Everything you need to know about Oxygen Sensor!

posted in Engine, Technical Info |

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The Oxygen Sensor is usually to blame for problems in the car performance and gas consumption. It’s stunning to discover that it’s hard to fail. I can count many of my friends who went to mechanics for car checkups and ended up changing the Oxygen Sensor. It seems that the mechanics knowing that the Oxygen Sensor is something mysterious to many people will make them believe that it’s broken and need to be changed, even with no obvious reason for that. In this article, you will find all what you need to know about Oxygen Sensor and even how to test it to determine if it’s working fine or not. So, enjoy it and please let me know what you think. Thanks Rick Kirchoff for the original post.



These procedures are only for self powered conventional sensors.


Some very new cars are using a different style sensor that is powered. *Many* Oxygen sensors are replaced that are good to excellent. *Many* people don’t know how to test them. They routinely last 50,000 or more miles, and if the engine is in good shape, can last the life of the car.


What does the O2 sensor do?


It is the primary measurement device for the fuel control computer in your car to know if the engine is too rich or too lean. The O2 sensor is active anytime it is hot enough, but the computer only uses this information in the closed loop mode. Closed loop is the operating mode where all engine control sensors including the Oxygen sensor are used to get best fuel economy, lowest emissions, and good power


Should the O2 sensor be replaced when the sensor light comes on in your car?


Probably not, but you should test it to make sure it is alive and well. This assumes that the light you see is simply an emissions service reminder light and not a failure light. A reminder light is triggered by a mileage event (20-40,000 miles usually) or something like 2000 key start cycles. EGR dash lights usually fall into the reminder category. Consult your owners manual, auto repair manual, dealer, or repair shop for help on what your light means.


How do I know if my O2 sensor may be bad?


If your car has lost several miles per gallon of fuel economy and the usual tune up steps do not improve it. This *is not* a pointer to O2 failure, it just brings up the possibility. Vacuum leaks and ignition problems are common fuel economy destroyers. As mentioned by others, the on board computer may also set one of several failure “codes”. If the computer has issued a code pertaining to the O2 sensor, the sensor and it’s wiring should be tested. Usually when the sensor is bad, the engine will show some loss of power, and will not seem to respond quickly.


What will damage my O2 sensor?


Home or professional auto repairs that have used silicone gasket sealer that is not specifically labeled “Oxygen sensor safe”, “Sensor safe”, or something similar, if used in an area that is connected to the crankcase. This includes valve covers, oil pan, or nearly any other gasket or seal that controls engine oil. Leaded fuel will ruin the O2 sensor in a short time. If a car is running rich over a long period, the sensor may become plugged up or even destroyed. Just shorting out the sensor output wire will not usually hurt the sensor. This simply grounds the output voltage to zero. Once the wiring is repaired, the circuit operates normally. Undercoating, antifreeze or oil on the *outside* surface of the sensor can kill it. See how does an Oxygen sensor work.


Will testing the O2 sensor hurt it?


Almost always, the answer is no. You must be careful to not *apply* voltage to the sensor, but measuring it’s output voltage is not harmful. As noted by other posters, a cheap voltmeter will not be accurate, but will cause no damage. This is *not* true if you try to measure the resistance of the sensor. Resistance measurements send voltage into a circuit and check the amount returning.


How does an O2 sensor work?


An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts.


The sensor does not begin to generate it’s full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor *will not spend any time at 0.45 volts*. In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer knows this is an “illegal” value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution.


The O2 sensor is constantly in a state of transition between high and low voltage. Manufacturers call this crossing of the 0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.


How can I test my O2 sensor?


They can be tested both in the car and out. If you have a high impedance volt meter, the procedure is fairly simple. It will help you to have some background on the way the sensor does it’s job. Read how does an O2 sensor work first.


Testing O2 sensors that are installed


The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedance DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.


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 your connections. Next, check your leads to make sure you won’t wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes 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 (> 0.45) near 0.45 or steady low.


Testing O2 sensors on the workbench.


Use a high impedance DC voltmeter as above. Clamp the sensor in a vice, or use a plier or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems.


ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don’t miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always *no* benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.




Wait for more from BMW E36 Blog.


Best regards,

Tony Sticks.

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There are currently 28 responses to “Everything you need to know about Oxygen Sensor!”

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  1. 1 On August 4th, 2007, steven said:

    can a universal oxygen sensor that has been wired up wrong cause the wiring loom to burn and fuse many wires together?

  2. 2 On August 4th, 2007, steven said:

    forgot to mention it was kwik fit that wired it together by bareing back wires and twisting them back togethter

  3. 3 On August 6th, 2007, Tony Sticks said:

    I think it’s unlikely that this should cause any fuse burning. Did you check if the connected wires are ok ? Did you test the Oxygen Sensor?

  4. 4 On December 14th, 2007, Ray Hill said:

    Hi Tony,
    I drive a 93 E36 325i and dropped it into my local BMW specialist garage for an Inpection II service that was due. The car was driving great, very smooth and quick. While they had the car they called and told me that I needed a rocker cover gasket fitted as the current one was leaking. I agreed to the work. I picked the car up later and drove it home. The performance was down and the car was stuttering at low revs.
    I brought it back and they ran some diags on it and revved the car very hard. They told me that I need a new O2 sensor. Having read your article on O2 sensors, I suspect that the garage damaged it when replacing the rocker gasket, yet they are looking at me as if I’ve gone mad. This is really pis*ing me off. If they have not used the correct gasket sealer does that mean that even if they replace the O2 sensor the problem will still be there?


  5. 5 On December 15th, 2007, Tony Sticks said:

    Hi Ray,

    I had to change the O2 sensor in my car and honestly didn’t feel any change in performance. So, I can’t be 100% confident that your O2 sensor is going to fix the problem or not. If you can see that the diagnosis result of the car is suggesting that the O2 sensor needs to be replaced (if you can see it yourself and not taking their word for it), I think you should replace it even if this is not the cause of the problem.

    Have your ever tried to run a diagnosis for the car before going to this garage?

  6. 6 On December 15th, 2007, Ray said:

    Hi Tony,
    No I haven’t run any diags before going to the garage. The garage was recommended to me by one of the guys who runs the local BMW car owners club, so I went along thinking they were the one that I should be dealing with.
    My big concern is that I dropped the car in for a service and paid for the service and got the car back runnning worse than when I left it in. I mentioned what you had said re. the sealant on a gasket change. They said that was nothing that could affect the sensor!.
    It looks like I will have to get the O2 sensor changed to see if we can get the performance back to where it was.
    I hope that works. One question for you Tony, assuming that they have used a sealant that has affected the O2 sensot, will the replacment sensor also be affected by this sealant?


  7. 7 On December 16th, 2007, Tony Sticks said:


    I don’t think the sealant has anything to do with the oxygen sensor. Maybe you had that O2 sensor gone bad long time before you took the car to the garage.

  8. 8 On December 17th, 2007, Ray said:

    Hi Tony,
    I only referred the the article above where is says

    “What will damage my O2 sensor? ”
    Home or professional auto repairs that have used silicone gasket sealer that is not specifically labeled “Oxygen sensor safe”, “Sensor safe”, or something similar, if used in an area that is connected to the crankcase. This includes valve covers, oil pan, or nearly any other gasket or seal that controls engine oil.

    As I mentioned, the rocker /valve cover gasket was replaced when the car was serviced so I was speculating that the sealant that they used could be a problem?. Maybe you’re right, the car is 14 years old, so perhaps it was on its way out anyway.


  9. 9 On December 17th, 2007, Tony Sticks said:

    That’s why I asked you if you’ve made a diagnosis for the car before. If you have done that and the O2 sensor was ok, it would mean that they have damaged it.

  10. 10 On March 1st, 2008, mehdi said:

    how can i clean my o2 sensor?

  11. 11 On March 3rd, 2008, Tony Sticks said:

    Hi Mehdi,

    I don’t recommend cleaning your O2 sensor, but if you’re going to, I suggest you read this first.

  12. 12 On March 31st, 2008, Jaime Quiel said:

    Hi Tony, I have an 328i 97, coupe (40,000 miles, new filters, new sparks). I bougth it almost 1 year ago, and after 1 or 2 weeks a check engine error blinks. I talk to a friend of mine to check it with the scanner it shows next code: “code 254, leak from gas tank”, but the point is that there is not leak. We clear this check engine code a lot of time and once again it appear after a couple of days, I had replaced the fuel filter, the gas pump, but nothing.
    I had consult to some BMW mechanic, but nothing, the BMW dealer says that they have to check a lot of things to find about the possible cause, that mean a lot of time and money. Maybe, maybe you or somebody could help me with that, please. The engine is working fine, I guess. Some times, just some times, when the engine is in ralenty, it had stop, for any razon that I don`t know. Bye, and Thanks.

  13. 13 On April 1st, 2008, Tony Sticks said:

    Hi Jaime,

    I did a quick search for you and found this:

    I hope this helps.

  14. 14 On April 11th, 2008, tony hdez said:

    hi, i have a 95 isuzu pick-up, it just turn to 90k milles. the o2 sensor lights up,should I change it? thanyou. Reynosa,mexico.

  15. 15 On July 3rd, 2008, BMW E36 M42 DISA Valve | BMW E36 Blog said:

    […] E36 318is idle and until now, I was unable to totally fix the problem. I’ve cleaned the ICV, changed oxygen sensor, checked for vacuum leaks but this didn’t solve my problem. I still have a problem in one of […]

  16. 16 On August 20th, 2008, Robert W. said:

    Hi Randy! I have been reading your articles and Answers on this site and they are quite helpful! Its a great thing you’re doing and I can attest to that! I have a little problem- I have a 1998 BMW 740iL with about 95k miles. I have put a lot of work into it. I change the oil regularly and just had some work done with the coolant, etc. I just recently brought the car to a foreign vehicle mechanic who indicated that my ‘check engine’ light was on because of the thermostat housing around the 02 sensor. He is indicating that with the model that I have the sensor cannot be changed singly. The whole entire housing around the thermostat would have to be replaced as it is damaged. He is unable to provide a reason why this would happen and it appears strange to me. I feel as if he just came up with that because he couldn’t figure out the ‘real’ problem. This is because the ‘check engine’ light was on last two weeks and then it disappeared..and then came back on this week. I’m confused as to whether thats really my problem or not. I live in TX and will be relocating to MI very soon. I will be driving the car out there and its about over 1k miles away. Does the 02 sensor and/or thermostat housing that this mechanic is indicating affect the operation of the vehicle? I know it affects performance but is it a ‘need’ to be replaced for the overal operation of the car? Do you recommend that I change it before the trip or it usually doesn’t cause the car to fail? Any feedback would be greatly appreciated!

  17. 17 On August 23rd, 2008, Tony Sticks said:

    @Robert W.: Hi, First my name is Tony, not Randy :p . Now for your problem, when the “Check Engine” is on. Do you get any error code? If not, I suggest you take the car to another mechanic to run a diagnosis for it and see what the error code(s) are. How did he know that the problem is in the O2 sensor? It’s possible that he’s right (since I don’t have much knowledge about your car), but I suggest you run the diagnosis and see the error code returned yourself before finding solutions for the car.

  18. 18 On November 18th, 2008, Cam said:

    Excuse me, i am installing an air-fuel ratio guage on my e36. I need to know what wire to tap into on my oxygen sensor. there are 3 wires. White, Black, and grey. I need to know which wire is the sending wire. Thank you!!!

  19. 19 On November 20th, 2008, Tony Sticks said:

    @Cam: The gray wire is the one that you need to tap into. It’s the one that provides the signal.

  20. 20 On January 26th, 2009, Tom Williams said:

    BMW E39 520i Feb. 1999.

    Vehicle misfiring, loss of power and stalls. I had the fuel system checked (tank cleaned, filter replaced, injectors cleaned and regulator replaced). While is an improvement overall, the same problem still occurs. At slow speeds or idle or slow speeds after fast driving, it starts to lose power to the extent that it does not accelerate even if you floor the pedal. Still misfiring in several cylinders and showing rich fuel in cylinders.

  21. 21 On February 6th, 2009, SCOTT said:


  22. 22 On February 19th, 2009, Sasha said:

    Hello everyone!

    Great article by the way! I have a 1997 328is and my check engine light is on with the code P0420 (P0420 Catalyst System Efficiency Below Threshold (Bank 1)). I’m not sure yet if my catalytic converter going out or if its just the mysterious O2 sensors. hopefully,

    Did anybody have the same problem before? How did you fix it?
    Does anybody know which side is bank 1 and which side is bank 2? or let me rephrase that, where are Bank 1 oxygen sensors located? any help would be great

    Thanks in Advance


  23. 23 On October 2nd, 2010, lee said:

    hi tony
    ive got a 96 e36 and ive just come back from the dreaded MOT test center thought the car was tip top as i had done alot work to i to make sure it passed but it faild on emisions.
    the HC was fine and the lamda reading was ok but the CO was way high can only be 0.30 % and mine was 1.17 on fast idle
    and 0.50 alowed for natural idle and mine was 0.71
    its got bosh super 4 spark plugs and new oil and oil filter new air filter and new elephant nose off the air flow metre and i added wynes oil aditive for liffters and engine cleaner and i added a fule aditive to clean cat smells and up fuel eficiency and lower emisions but nothing has worked what can i do to bring the carbons down?

  24. 24 On February 11th, 2011, Eddie said:

    hi tony.
    i have a 1994 bmw 740 iL and its running rich.
    i changed the coil packs sparkpluga n map sensor. and it just recently started running rich it has lost alot of horse power.
    the engine runs rich and shakes when its on park.
    but as soon as i start driving it it starts backfiring as i take off and it keeps doing it as soon as the rpm goes down when it shirts to 2nd gear.
    when i start it in the morning, cold its the worse it keep wanting to turn off.
    could this be the problem?

  25. 25 On August 12th, 2011, jordan said:

    I have a 2007 bmw 328i sedan & I need to change an O2 sensor. Can someone please let me know how long should this job take a seasoned mechanic to complete? Based on local labor costs, I can back into the total labor costs. Not going thru a BMW shop. Thanks.. Jordy

  26. 26 On August 27th, 2011, Oz said:

    E36 96″ 328i ……need to change the oxygen sensor. Diagnostic tells me bank1 sensor 2. Which one would that be? please help!

  27. 27 On October 18th, 2011, chet gray said:

    Where are the oxygen sensors, there are two located on my bmw 2007 328i 4 door sedan. And how to change them. Thanks Chet

  28. 28 On April 4th, 2012, DAN MASANGO said:

    where is the oxygen senssor of the bmw e-36 located?