O2: what do it do? (O2 delete?)
 

So dumb question, but with my de-cat causing O2 CEL is there any reason to even have a downstream O2 sensor? I'm gonna mask the code anyway, so can the upstream act solo, and the downstream be essentially deleted without causing the car to limp? I suspect my downstream unit is toast, so if I can just unplug it without ill effect I'll do so.

The rear O2 has some minor effect on close loop fuel accuracy when the car is in a stable cruise, and it can be used to pull the AFRs back in line if the front O2 were to start sending false signals to the ECU.

Otherwise, no it isn't critical at all, and can be unplugged and removed. They usually fail fast on catless midpipes.

If the usual cat delete 0420 code keeps popping up that means it's still working, right?
How can one tell when it doesn't work anymore?

There are 9 different codes that are triggered from the rear O2 sensor, i think 4 of them are direct O2 sensor failure codes, with at least 2 more possible with a rear sensor failure. P0420 appearing generally means that it is working, but not explicitly. It's possible that it is just in the progress of failing, and failing the cat efficiency test without failing any of the others yet. There isn't a predefined pattern of exactly how they will fail every time. I've seen a few different ways, and I doubt I've seen them all.

It's most easily identified in the mode6 test data though, as it will fail any one of the tests that are run, even before the ECU decides to use those failures to throw one of the failure codes.

Thank you.

Guys, this is really informative, thank you.