What does an 02 sensor do?
Thread Starter
Instructor
Joined: Apr 2009
Posts: 115
Likes: 1
From: Gurnee IL
What does an 02 sensor do?
My mechanic is recommending that I ad an 02 sensor to the list of items to buy as we are going to upgrade the exhaust on my recently purchased C5. Can someone give me an idea what this does? I don’t mind the addition, its only a $40 part, but I like to understand why im doing something, and his explanation doesn’t make any sense. To much technical info, and im not a gear head.
I have already googled it, but am looking for a simpler explanation.
***Also, since this is a 10 year old car, feel free to recommend anything you think I should check, test, or replace for optimum engine maintenance/performance. Im scheduled to replace the spark plugs and wires, and get a general tune up this weekend.***
I have already googled it, but am looking for a simpler explanation.
***Also, since this is a 10 year old car, feel free to recommend anything you think I should check, test, or replace for optimum engine maintenance/performance. Im scheduled to replace the spark plugs and wires, and get a general tune up this weekend.***
Platinum Supporting Vendor
Joined: Sep 2001
Posts: 3,665
Likes: 340
In simple terms, it is a sensor that reads the exhaust content. It lets the ECM know what they air fuel ratio is of the engine and can make adjustments to the fuel trims to make sure the car is running at its optimum.
Burning Brakes
Joined: Nov 2006
Posts: 989
Likes: 8
From: Chino Ca.
I am sure he didn't recommend that you only replace one. There is one for each bank. 
EDIT: Actually there are two on each bank, but the rears are hardly ever replaced.
EDIT: Actually there are two on each bank, but the rears are hardly ever replaced.
Last edited by DIXZ; Apr 23, 2009 at 03:07 PM. Reason: Correction
Le Mans Master
Joined: Nov 2005
Posts: 6,993
Likes: 25
O2 sensors are normal wear items (like plugs/wires/..)-they typically get "LAZY" over time and do not respond quickly enough well before they fail and throw codes. The front O2's really do the work of telling the ECM how much O2 is in the exhaust stream, the rear O2's measure O2 out of the converter to measure catalyst efficency. The rear ones usually have heaters too since they are so far downstream and O2's have to get quite warm to function. When you graph the O2 output you can see the continuously from rich to lean
Melting Slicks
Joined: Jul 2000
Posts: 2,541
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From: Chicago
They simply monitor your engines exhaust gasses to check for the proper proportion of air to fuel and continuously feed data to your cars computer which it uses to maintane that ratio. That being said, I wouldn't replace them unless they are throwing a code ie bad. No sense in replacing a functioning sensor.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
The actual O2 sensor works by converting oxygen content in the exhaust and heat into a small electrical DC voltage via a catalyst inside the O2 sensor (the operating voltage is 0.0 VDC to 1 VDC) That voltage is used by the PCM to control the closed loop air fuel ratio.
The PCM normally operates from .2 VDC to .9 VDC.. As the PCM reads the output of the sensor, it increases or decreases the fuel injector operating frequency to allow the sensor output to toggle low to high, low to high etc....... by varying the injector pulse width.
So when your engine is in closed loop, the fuel injector should normally cycle and cause the O2 sensor to vary from around 250 mv to 850 mv. It can see more or less during extreme conditions like accelerating or deceleration.
Bad sensors don't cycle properly and hang out in the middle 450 ish mv range and cause poor part throttle performance and fuel economy.
If you can find some one with EFI Live or HP Tuners, you can have them log your engine and observe the O2 Sensor/s operation.
The PCM normally operates from .2 VDC to .9 VDC.. As the PCM reads the output of the sensor, it increases or decreases the fuel injector operating frequency to allow the sensor output to toggle low to high, low to high etc....... by varying the injector pulse width.
So when your engine is in closed loop, the fuel injector should normally cycle and cause the O2 sensor to vary from around 250 mv to 850 mv. It can see more or less during extreme conditions like accelerating or deceleration.
Bad sensors don't cycle properly and hang out in the middle 450 ish mv range and cause poor part throttle performance and fuel economy.
If you can find some one with EFI Live or HP Tuners, you can have them log your engine and observe the O2 Sensor/s operation.
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Tech Contributor
Joined: Dec 1999
Posts: 32,910
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
You can (normally without any issues, remove the sensors and re-install them in te new exhaust.(if there good). You DO NOT just have to get new ones if your replacing exhaust. 
Bill
Bill
Race Director
Joined: Mar 2000
Posts: 17,602
Likes: 20
From: Palm City Florida
And I'd like to point out that bad sensors dont always set Check Engine Light..A lazy sensor causing the motor to run rich can easily be hidden by the Cats working overtime and cleaning it up by the time the 2nd O2 sees anything is wrong..
How do I know? my gas mileage plummeted.. but no CEL was lit.. I cleaned the MAF and the air filter... to no avail.. Added a bit more air to the tires.. No help..
I noticed while walking behind the car when it was running in the garage that it stunk.. Strong hydrocarbon smell.
I decided to replace the O2's. Immediately after I replaced them I went to "smell" the exhaust.. It smelled like warm humid air.. thats it.. no eye burning smell..
The gas mileage immediately came back to normal and is still consistant after a dozen tanks of gas..
