What's a P0420 code?
Isn't it Catalyst Efficiency Low Performance, or something to that effect.
I started getting mine when I installed LT's and my catted Y-pipe. I figure the cats in the new Y-pipe aren't as efficient as the factory cats. (They are much smaller, so it makes sense.) The Y-pipe also had bungs for the rear O2 Sensors so I installed them. I suppose the O2's are reading more emissions and throwing the code.
When I finally get my Mail-Order-Tune I'm sure the code won't show up again.
Do you have LT's? If so, I'd say its time you consider getting a Tune as well.
Moral of this story: It could be a wide variety of things causing the code...don't assume it's a cat just because it says "catalyst" in the description. I could have spent $100 or so trying to fix the code, but instead it cost me under $1.

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Do I need to clean the tubing? 






