o2 sensor

[probedude]

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.

It does affect mileage and on my cars it has affected driveability.  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.

Dave