I scanned an OBDII car tonight for the first time and something caught my attention. The rear O2's were barely moving at idle (both sides in the 850mv range) and weren't cycling very much at all when driving (both sides in the 650-900 range). This is something I've not looked at before and was wondering if it's normal for the rear O2's on an OBDII car to stay statioary like that. Stock exhaust with cat-back, factory cats. Thanks.

 

That sounds pretty normal to me.

 

Ok, thanks for the info. I just wasn't sure what to expect out of those rear O2's.

 

When the rear O2s start to fluctuate and read like the front O2s, that tells the PCM that the cats are no longer doing their intended job. Then you'll get a SES light.