Bad O2 Sensor?

Which 2 were at .28 V? If they were post cat this may be OK. Pre cat would not be OK.

P0420 - Catalyst System Low Efficiency (Bank 1 Sensor 2)

This would indicate that sensor 2 on bank 2 was too active, not that it was inactive.

Actually, the upstream sensor that's doing the cycling from .1 to .9 V has the downstream that is "stuck" at .16V. That's on the front bank (bank 2?).

I'm confused as hell as to what bank is what. The other bank had one that measured .28 and the other 2.5V. Something seems wrong there. That's the bank on the firewall which I assume is bank 1 (#1 cylinder).

It looks as though there are 3 cats on that model. The front pipe has a sensor at the top (upstream) and one at the bottom (downstream?).

Then on the other bank there's a sensor at the top near the exhaust manifold and one down and mounted in the cat (downstream?)

Sensors are numbered from the engine back from an exhaust manifold.

Sensor 1X is always on bank 1 (the same bank as cylinder 1).

.28 and 2.5 V steady? That would be a problem.

And 2.5 V on an O2 sensor would be a real problem. Anything over about

Upstreams would be X1, downstreams would be X2 if you have 4 sensors as it sounds.

Actually it turns out that the 2.5 V reading was from a connector that went to something else. Oooops...I knew that was wrong.

I got under it last night so that I could physically see and know what connectors go to what sensors.