o2 sensor

[Steve Foltz]

Dave wrote:
I think the first O2 sensor in the exhaust is the one the computer uses to
adjust the air fuel mixture.  The second one after the catalytic is just
there to let the computer turn on the MIL light if it doesn't detect a
working catalytic.

That is true and if either is not functioning properly on a OBDII car it
will pop a MIL light.  When my front o2 went out the car seemed fine.  What
can happen is the two sensors readings don't stay within a predetermined
range (they should be similar for some factors) over a certain time (such as
two times in 10 minutes or twice during two separate drive times) and it
will pop and MIL.  It could still be working 99.5% of time.

It does affect mileage and on my cars it has affected drivability.  If your
gas foot only goes fully up and fully down, you may never be at the point
that the ECU cares what the O2 sensor is reading.

I think the O2 only works at part throttle (not sure) but every car has to
idle at some point and then the o2 sensor readings get compared.

Jon wrote:

Again, I'm going to bet that it's the one topside, since the function
(taking another big bite of my banana) of the post-cat O2 sensor is
mostly to make sure that the cat is functional.  The one on the manifold
is the one that checks mixture and tells the ECU to enrich or trim the
fuel supply, thus affecting your mileage.

I concur.  I would only add that the mileage was fine on my car even with
the MIL light.  If my 02 sensor was worse mileage would have gone down.  The
two O2 sensors do have a self diagnostic built in not only for the cat but
also the sensors themselves.

I get this info out of my FSM and conversations with a Nissan mechanic who
was a mountain bike riding buddy of mine.

Steve