So, my car's been through a few entire I/M readiness cycles, and just twice the CEL has come on with P0420. Both times it's been on a highway in the rain. I'm seriously thinking that the problem is not a cat problem, but an O2 sensor problem.
I'd like some other opinions, though. What would cause an apparently good exhaust to say that there are cat efficiency problems in the rain?
I took a look at the sensor itself (didn't take it out), and the wire seems loose but not broken I don't think... not sure how to tell if it's bad without replacing.
Anything I can check, datalog, take pictures of, etc. to diagnose this problem? I'm probably going to do the mechanical CEL fix later on when I go nearly or totally catless, but I'd like to make sure that there's not a problem with the sensor or its wiring...
Any ideas appreciated.