P420...Which Cat? P1831?
 

Computer replaced and still throwing p0420, which catalytic converter is this? Is it the one that comes off the forward bank near the oil filter? Is this a pretty simple unbolt the old and bolt in for the new one?

Also, now getting code P1831, transfer case counter clockwise shift relay coil short circuit to ground...Huh?

P0420 is thrown when the up stream and downstream O2 sensors are showing values too close together. This condition (catalyst efficiency) may indicate a bad cat, or a problem with one (or both) of the O2s/connectors/wiring. I'd check those first before ripping a the cat off (they're not cheap!). BTW, the code indicates a problem with Bank 1. Good luck!

I checked my JTIS and P1831 isn't in there. Mine only goes up to the 2004.25 model year so maybe this is a later code added to the DTCs.

Looks like this computer may be bad as well, the 831 code has something to do with the clutch pedal and our car does not have a clutch pedal.

As far as the cat goes, I am finding a bolt in cat for about $280, thoughts on that?

Good price on the cat. Be thankful you don't live in Cal. CARB certified cats here are about twice as much as 49 state cats. Also, if you replaced the ECU and are getting a clutch pedal code I'm thinking the replacement ECU you installed is from a manual car. Ignore it.

Thanks for the response. Turns out the cat was $444.44 shipped and I wish I could ignore the code but it won't pass inspection with the CEL on. Ill report back when the new computer and cat are installed.