O2 Sensor(s) Question

[rnjmur]

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.

So I looked up the engine engineering specs for post cat O2 sensors, and you are mostly right although I wasn't completely off. Because of the storage of O2 in the catalytic converter, downstream exhaust reads slightly richer and should actually fluctuate from .35v to .7v (at closed loop when the cat is at operating temperature) but with fewer changes.

OK, here's a question for any Cobalt/ecotec owner and/or tuner; the PCM will test your cat's efficiency (P0420 CEL) whenever at idle with clutch depressed for several seconds(manual) or idle car in gear (automatic), by first making the AFR richer by about 1 unit; i.e. from 14.7 to 13.7, and then lean by about the same and monitoring the downstream sensor output. If you log the data at this time you'll see it; so what kind of voltage are you seeing on the downstream sensor during this test?

The test is not so much about what the voltage is during the test. When the PCM conducts the test it is checking to see how the timing of voltage switches are taking place. This is what is going on:

Oxygen Storage Capacity (OSC) test is where the PCM takes control of the mixture to measure the time-to-capacity for the converter.

The mixture is held rich to empty the converter of O2, and then switched lean. The upstream O2 will quickly switch lean, but the downstream O2 will not switch until the converter has absorbed all of the O2 it can and the extra exits the substrate.

[Rush]

So it sounds like it is measuring how long it takes to absorb all the O2 it can, so a slow response would probably be good in this case. I guess that explains why the rich cycle takes longer than the lean cycle; either that or it fails right away and stops.
Thanks