The rears are used to measure the effectiveness of the cats.

 

Ok, that makes sense. So what kind of adjustments will the PCM make based on the readings from the rear O2s if not A/F?

I'm wondering if it's woth it to replace all 4 O2s, or just replace the front 2 and leave the rears as is ....

Thanks,
Thomas.

 

I have 130k on the orginal 1 o2 sensor and I have a used 2nd o2 sensor, car seems to run fine though, I wondor if changing them would help?