O2 sensors

[Lemay88]

Bonjour guys,

Me again with a noob question.
I know our cars have 4x O2 sensors. 1 on each side in the front that tells the pcm the fuel mixture to use. 1 on each side in the rear,,,those are the ones I am not sure what they are for.
From what I got as info is that they are there to confirm the emission test and do not really tell the pcm what to do...is that assumption correct??
thanks again for your input

Ben

[Blue96GTS]

Here's my answer, the more technical version being on page 25-8 of your service manual:

The rear O2 sensors are there to assist with determining if your Cats are working correctly. The upstream O2 sensors are for fuel mixture, as you point out. After the upstream sensors are (supposed to be) your cats, which are there to burn off any remaining fuel in the exhaust. The downstream O2 sensors should then be reading "clean air", indicating all things are working correctly.

[Lemay88]

Great, pretty much what I thought.
Second question...
Do they interact in the fuel mixture??

[daveg]

Post O2's have no affect on fuel metering. As stated, its there to monitor Cat efficiency.

[Lemay88]

Ok, thanks