Code P0420 on 2016 LTZ chevy silverado

[Sambo Lester]

I put an aftermarket cat on and still getting 420 code

[CamGTP]

You could try spacing the rear o2 sensor out of the exhaust gases a little bit to see if that will stop tripping the code.

 

I always used to call in the spark plug non fouler trick because those parts are the same thread pitch as the o2 sensor bung and it's a cheap thing to try. You can google it to give you a better idea of what I'm talking about.

[Hawaiian Brokechanic]

On 6/11/2021 at 2:27 PM, Sambo Lester said:

I put an aftermarket cat on and still getting 420 code

I just when through the same issue with my Silverado... I would check exhaust leaks at the flanges and to the muffler and repair to start.  If no leaks, check down stream O2 sensors for excessive carbon and clean.  If bad, replace.  If P0420 persists, you may have a bigger issue with a bad lifter or a leaking exhaust manifold.