what do the rear most O2 sensors do in my 2002 chevy Avalanche?

[DanN]

My check engine light comes and go and the scanner is telling me thats my problem.. should i change them?

 

[hyde]

If they're after the catalytic converter, they monitor the converter to make sure it's filtering the exhaust properly. It is very unusual for them (you should only have one) to fail. It could be the catalyic converter (again, rarely do they fail that soon) and is probably not the sensors themselves.

If it's the ones before the converter, they control the fuel mixture and usually start to fail around 75k miles or so.