when does the computer listen to the rear o2 sensor? (the one farthest from the engine) if it dies do you lose performance/gas mileage?

 

Im thinking all the time and yes.

 

i remember a thread a while ago when it was stated that the computer only uses the front two sensors to adjust the air/fuel ratio (performace/gas mileage) i just want to know if that was a true or not....

 

The rear O2 only monitors the Catalyst operation.
The ECU monitors it constantly after initial warm up.

 

The ecu does not constantly watch the o2's. Only when they get to operating temps (which is around 500 or so) I forgot what it's called but we run on some other system until the o2's get to the right temps.

 

AFAIK, the rear O2 sensor has no effect on performance. It monitors the CAT as njmaxseltd said. Mine has been on the blink for years, it was replaced once when I bought the car used in '97. It failed a few yrs ago & I never bothered to replace it. My performance is great & so is my mileage.

 

The computer will only rely on the rear o2 sensor if one of the front two go out. If your rear goes out and the front two are still good it will not hinder your performance.