Originally Posted by
gmtech16450yz
Post cat O2 readings... As I said in my post above (sometimes I wonder if anyone reads what I post. I often think people's opinions of the average dealer tech is so bad I should have never used "gmtech" as my screen name. I'm NOT your average dealer tech and I know a lot of them are dumb#sses.) post cat O2 readings can be ANYWHERE between 0 and 1 volt and be perfectly normal, and does NOT mean the cat is no good. What the ECM is looking at to determine if the cat is good is the frequency of rich/lean transitions. Think about it, the pre cat O2 sensors switch between rich/lean very fast and constantly. If the cat weren't there or working, the post cat O2 would also switch between rich/lean very quickly and constantly. Actually, having the post cat O2 sitting at 0 or .1 or .2 volts isn't that common, even at idle. It's completely normal for the post cat O2 to be reading .6 or .7v at idle or cruise. The only times you can really say when it's "supposed" to be reading a certain value is at WOT/PE, where between .85 and 1 volt is normal, and when you're in DFCO, where it will go full lean. (Usually.)
So, to answer the OP's specific question, which I don't think anyone did...
It could be anything from .2 or so to .7volts. Rarely 0 or 1 volt. If I had to say what I've seen most of the time at idle I'd say sometimes .25 and sometimes .65.
Sorry if I came off as a bit of a d!ck, just saying if you're not 100% sure you have the correct answer, you probably shouldn't post. Misinformation seems to spread faster than truth.