P0420 is a catalyst efficiently code, which is triggered by the rear 02 sensor not the front 02. Which means the cat is bad. How did they check the cat? without pulling it and looking through it you really can't tell. We had a Pontiac G6 in the shop running about like the way yours is described and looking in the inlet flange of the cat it looked ok...when we cut it out and looked through...you could see that it was half clogged. Have you tried running it with the front 02 removed? It will be a bit loud but it will give the exhaust an escape route, if it gets its power back, there is a restriction in the exhaust....don't run it like that to long...just to test it out....

 

oops...my mistake, wrote that down from the first error code...prior to replacing downstream O2. Now it is showing Bank 1 on the diagnostic, which I have replaced 2x.

 

What code is it? If it is a to lean or rich then it could be a vacuum leak or a clogged injector or an injector stuck open for rich. That would be a problem that replacing the 02 would not fix.

 

Sounds like a headache. Trade if for a 99+ silverado and be done.

 

I guess I was confused...I was under the impression that Bank 1 referred to the upstream O2. Now I know that P0420 Bank 1 refers to the downstream O2. Either way, the CEL is still on, and I am going to clear the code before I head to the Dealer to trade it in on a more economical, practical vehicle. Thanks for the great advice!

 

Are you by chance using bosch sensors? If so that's your problem

 

The downstream is a Standard brand. Put a Standard in the upstream, truck would not idle....replaced it with a Denso. Keep saying that the downstream Standard needs to be replaced....