[maxwedge]

I don't see code p1000, some posts here seem to doubt the complexity of the p0420, if they were in the repair industry and saw the amount of cats being replaced and the code coming back they would be quite enlightened, that being said, GMMerlin's post on the code is on target. Possible coolant damaged the cat but only a graphing scanner or oscilloscope to compare pre and post cat sensors, or checking the inlet vs outlet temp or a warmed up cat will confirm if it is bad, there should be 100-150 degrees difference from the inlet to the outlet. I'd clear the code and see if it come back first.