I have had issues with the cat CEL (P0420). Someone correct me if i'm wrong, but the code is thrown when the 02 sensors don't read the value they are expecting. So the culprit must be the cat?
Well, I just changed out my coolant temperature sensor (the one thing I forgot to do when I rebuilt the cooling system last year
) and that turned off my P0420 code. I guess the temperature the cats were expecting was getting skewed by the bad temp sensor.
Anyway, P0420 is a pita CEL and it can be the cat, 02 sensors, or any number of things apparently.