How does a Oxygen sensor ACTUALLY work

[Mechanicboy]

I've tried finding this through google, but they all just say crap like "The mechanism in most sensors involves a chemical reaction that generates a voltage" What chemical, oxygen?

What I'm getting at is what is measured differently between a pre and post cat O2 sensor, does it measure the oxygen count, the CO2 count, both? How does it measure it, does it cool the element to change resist similar to a MAF sensor? How does it make a voltage or resistance change?

I hear all sorts of ways people try to trick O2 sensors into believing cats are there when they are but that's not what I'm after. I want to understand how exactly oxygen sensors work so I can diagnose a bad sensor better and understand how tricks like the "non-fouler" work or don't work.

Maybe this question sounds stupid, but I still want to know.

 

[joegr]

It monitors oxygen, specifically O2.
The chemical reaction is between the O2 in the air and the sensor electrodes.
The sensor has to be hot (very hot) to function correctly, however it does not measure temperature, and its reading are not altered by changes in temperature so long as the temperature is high enough to operate.

The mixture control (front) O2 sensor switches rapidly between rich and lean as the air pulses go by. Mixture is adjusted so that it shows lean or rich the correct percentage of the time.

The monitor O2 sensor (back) switches more slowly than the front one because the converter removes the higher O2 pulses. The check engine light will be triggered if the back O2 sensor signal becomes too much like the front one. It's also triggered if the back O2 sensor doesn't switch some or goes too high or too low in voltage.

 

[milehighmikey]

It monitors oxygen, specifically O2.
The chemical reaction is between the O2 in the air and the sensor electrodes.
The sensor has to be hot (very hot) to function correctly, however it does not measure temperature, and its reading are not altered by changes in temperature so long as the temperature is high enough to operate.

The mixture control (front) O2 sensor switches rapidly between rich and lean as the air pulses go by. Mixture is adjusted so that it shows lean or rich the correct percentage of the time.

The monitor O2 sensor (back) switches more slowly than the front one because the converter removes the higher O2 pulses. The check engine light will be triggered if the back O2 sensor signal becomes too much like the front one. It's also triggered if the back O2 sensor doesn't switch some or goes too high or too low in voltage.

That said, good luck in devising a test to see if it works right. Best advise I would recommend would be to suspect the rear sensor in most cases of cat efficiency monitor tests failing. I am 3 for 3 in eliminating MILs pointing to bad catalytic converter efficiency results. Thankfully, the converter is good if it is always taken care of. not sure if this pertains to diesels though, in case you are actually looking to improve diagnostic times at your job.

 

[rudy3285]

Oxygen sensors have two electrodes, usually made of yttria stabilized zirconia. One measures the gas of interest (oxygen in the exhaust) and the other measures a reference gas--so the computer can know where the exhaust gas should be at. It compares the potential between the two electrodes to know if the oxygen in the exhaust is at the correct concentration.

 

[Mechanicboy]

It monitors oxygen, specifically O2.
The chemical reaction is between the O2 in the air and the sensor electrodes.
The sensor has to be hot (very hot) to function correctly, however it does not measure temperature, and its reading are not altered by changes in temperature so long as the temperature is high enough to operate.

The mixture control (front) O2 sensor switches rapidly between rich and lean as the air pulses go by. Mixture is adjusted so that it shows lean or rich the correct percentage of the time.

The monitor O2 sensor (back) switches more slowly than the front one because the converter removes the higher O2 pulses. The check engine light will be triggered if the back O2 sensor signal becomes too much like the front one. It's also triggered if the back O2 sensor doesn't switch some or goes too high or too low in voltage.

So rich = right voltage(say 0.8v) and lean = low voltage(say 0.2v). The front sensor will switch between these and the rear sensor(post cat) the switching voltage should be less so show that the cat is working and stabilizing the oxygen count and keeping the voltage more stable?

It's also triggered if the back O2 sensor doesn't switch some or goes too high or too low in voltage.

Will this set a P0420, catalyst efficiency low? Is OBDII not very good at detecting bad sensors?

That said, good luck in devising a test to see if it works right. Best advise I would recommend would be to suspect the rear sensor in most cases of cat efficiency monitor tests failing. I am 3 for 3 in eliminating MILs pointing to bad catalytic converter efficiency results. Thankfully, the converter is good if it is always taken care of. not sure if this pertains to diesels though, in case you are actually looking to improve diagnostic times at your job.

You getting P0420/P0430? You had three bad senors and all three times it said it was the cat efficiency? Did you replace the pre or post catalyst sensors? Is OBDII not very good at detecting bad sensors?

Oxygen sensors have two electrodes, usually made of yttria stabilized zirconia. One measures the gas of interest (oxygen in the exhaust) and the other measures a reference gas--so the computer can know where the exhaust gas should be at. It compares the potential between the two electrodes to know if the oxygen in the exhaust is at the correct concentration.

Is the reference gas the outside air? Could the reference circuit/gas/area get contaminated causing a false reading of the oxygen in the exhaust stream?

 

[milehighmikey]

I never had a pre-cat sensor die, always the after catalyst sensor.

 

[LS4me]

I never had a pre-cat sensor die, always the after catalyst sensor.

My mechanic tells me the opposite. The rear (monitor) sensors rarely fail. When I had the low efficiency codes it was the front two.

 

[Mechanicboy]

I never had a pre-cat sensor die, always the after catalyst sensor.

My mechanic tells me the opposite. The rear (monitor) sensors rarely fail. When I had the low efficiency codes it was the front two.

I think both could be right. If sensors "slow down" in response then replacing the pre or post will change the response of the voltage wave to the ecu, thus it will read a greater difference which might be enough to satisfy the ECU into thinking the Cat is good. For both you guys, were these experiences on the LS, other cars, or both?

I just wish there was a easy way to test them without a scope or data logger. If you had a live data stream through a laptop then you could watch response times of both and determine which to replace. It both were marginal I'd probably replace the pre-catalyst just for the fact the it affects A/F mixture more.

On a side note, the major manufactures have admitted that the post-catalyst DOES affect the A/F ratio a little bit. Why? They say that adjusting the A/F ratio slightly improves emissions by making the catalyst operate at peak efficiency which isn't always the optimum ratio for the engines A/F ratio. They all claim that it's only a very small percentage of the fuel trim, less then 2% I believe.

On a second side note, check out my other post, this is what brought my curiosity on.

http://www.lincolnvscadillac.com/showthread.php?t=73747

 

[LS4me]

For both you guys, were these experiences on the LS, other cars, or both?
http://www.lincolnvscadillac.com/showthread.php?t=73747

Mine was my 2000 LS. Come to think of it..... I did end up replacing the rears also. For their low cost, I would just replace all 4; especially if you have over 100K on the engine. IIRC, they're about $40 a pop.

 

[Mechanicboy]

Mine was my 2000 LS. Come to think of it..... I did end up replacing the rears also. For their low cost, I would just replace all 4; especially if you have over 100K on the engine. IIRC, they're about $40 a pop.

I've replaced the fronts on my LS already, thinking I was running rich, which I wasn't. All my problems Have been coils, but hopefully that's solved once and for all with my Accels. Sh1t, I need to finish the writeup I'm a procrastinator, sorry.

 

[cammerfe]

The best answer is to use an SCT to eliminate the before after function and relegate the sensors to input for fuel mixture. Throw the cats in the corner---or take them to the bone-yard and sell them.

KS

 

[Mechanicboy]

The best answer is to use an SCT to eliminate the before after function and relegate the sensors to input for fuel mixture. Throw the cats in the corner---or take them to the bone-yard and sell them.

KS

I'm in California, which as everyone knows has the sh1ttiest smog laws(there's others too but lets focus) so I have to deal with it. I'm trying to get a better understanding of oxygen sensors, not just LS specific. Actually what brought this on is this: http://www.lincolnvscadillac.com/showthread.php?t=73747

I'm trying not to spend much money because I'm not going to keep it long.