So, got the P0420 today. Stock FXT, 101k miles w/AP stg 1 map, OE dp and Milltek 3" CBE. No other engine mods. I know O2 sensors go bad, and I know the cats can go bad too. Any easy way to tell the difference?

I know I can just tune it out in AccesstunerRace, and I do plan to go stg2 with a catted DP, but I've been told that the 05+ ECU's use the second O2 sensor in the fueling strategy. If it is a bad cat, is there any problem putting miles on it? I still have good exhaust out the tailpipe, so I don't think the catalyst is plugged or blocked.

Anyone have a 3" catted DP they need to get rid of?