Code P0420 WTF ??

I have an '02 LeSabre with 140K on it, and the last month I have my "Service Engine Soon" on and the code is P0420 which is a pain in my asss because it could be this or it could be that. So my question is, which is it??? Is it my Cat converter or oxygen sensor or do I just drill a hole in my exhaust pipe past the cat but before the oxygen sensor ??? Anyone please help..Thanks.

Rob

Wouldn't surprise me if it was the cat.

Google or do a site search for plenty of info, the cat is not always at fault. The drilling a hole fixes nothing including the code.

Yeah...i was just throwing that out there. I've read up on it and found out it could be the oxygen sensor and not the cat at all. Oxygen sensor has to be cheaper than a new catalytic converter but if it's the cat than I wasted my $$ anyway.

Rob