So, I'm new here, and yes I did a basic search and I get the idea that these cars (mine is 2002 si with 192k) seem to go through catalytic converters over time. Mine is apparently on its third converter and is now again throwing a p0420 any time the OBDII cat test completes, which sometimes takes as long as several hundred miles, but usually it's less. I am attempting to sort this out and I have some quick questions:

BLUF (Bottom Line Up Front, for those that know)

1.Is the upstream sensor on a 2002 Si an oxygen sensor or an "air fuel sensor"?

2.Is what an OBDII scanner (using Torque app for Android) labels O2S11 always, really, the upstream sensor and O2S12 always the downstream?

3.Is it possible that some moron swapped the plugs so the ECU sees upstream and downstream reversed?

I ask because the data shows O2S12 ocillating regularly at idle, going lean on closed throttle, etc. exactly as I would expect from the upstream sensor, and O2S11 mirroring O2S12, but with a much smaller amplitude and some delay, staying more or less even at idle while O2S12 oscillates rich-lean, exactly like you'd expect from the downstream sensor on a good cat.

If swapping the plugs is impossible, then my guess is that the cat is in fact bad, and O2S11 simply looks odd because it is really an air fuel sensor being reported by OBDII as an O2 sensor because that's all OBDII can do.

For anybody that wants the whole story, because diagnosing codes does not occur in a vacuum of other relevant information:

The car also burns oil, so a bad cat would not be a surprise. The PO, a close friend (I am actually sort of baby sitting his car while he lives in Hawaii indefinitely, maybe I'll give it back, maybe not, we'll see) tells me it leaks, a lot, from the valve cover. I look at the valve cover and under the car, and he's full of crap. Yes it is weeping a little around the valve cover, but the oil is being burned, not leaked. Plenty, maybe 1 qt every 2 tanks of gas--driveable, but clearly something's up. Exhaust tips black, soot on bumper, but no visible smoke. I suspect problems with PCV system, notice that the upper hose from the valve cover to air cleaner housing has separated (well thar's your problem!). I've corrected that and installed a new PCV valve because cheap. We will see if this corrects the oil burning problem, probably should do compression test, but that will have to wait until I have a few more minutes to fuss with it...

If it turns out that his rings are actually that shot, I'll be less likely to replace the cat, if it's really bad, at least certainly not without rigging some sort of catch-can in place of the original PCV system.

I know this is not an uncommon problem--anybody got some quick answers?