2008 SIlverado 4.8 P0420 Code but has good Catalytic Converters

[swathdiver]

Did you try swapping O2 sensors from bank to bank?  Really though, when a P0420 is thrown, it's the cat.

 

Even the wagons could have had the 4.8 too in the lower trim level Tahoes and Yukons from 2007 to 2009.

[evad]

it probably is a cat ...but sometimes you have to wonder what the cause is other than the symptom.....ohh the 4.8 thing was an inside joke for my buddy on the forum ... i guess he didn't wantto chime in

[evad]

..p0420 is generic ,week spark ,exh. leeks even o2 sensors seem to get week after so many miles ' all can cause fuel to be dumped in a cat .i do feel it helps to put everything back to spec before we read into the code to much .When you find out what it is please let us know....im very interested

[Brandon Longerbone]

So here's an update. Over the weekend I decided to tackle the exhaust manifold bolts. Figured they needed to be done anyway. Unfortunately, the rear most bolt broke off about an 1/8 inside the head, so I ended up having to take the head off, doing a head gasket and intake manifold gasket. Even replaced plugs and wires again since they had to come out anyway just to be safe. Not to mention the exhaust manifold and catalytic converter gaskets.

 

Good news is it fired right up, runs as good as it ever has, and (having cleared the code before hand) the code hasn't come back up yet. I guess I'll be back if it does... Thanks for the suggestions.