I have a 2015 Buick Encore that keeps telling me I have a check engine light for the same code P0420.I have replace all of the following items...intake manifold, pcv, valve cover gasket, both Bank 1 & 2 sensors, the flex pipe and the cat...the light will go away for a day and then if you drove it for and hour the light comes back on...I am beyond frustrated 😠..car runs great to drive crazy with the engine light...HELP!!!

 

P0420 is for "catalyst efficiency". Which means the front and rear oxygen sensors (on the converter!) are indicating the converter is not working as well as it should. The engine-related oxygen sensors keep the engine running as it should.

Replacement cats are not all created equally or work as well as the OEM production cats do. A few times, our dealership customers opted for the "muffler shop" cats, as they were cheaper, but they kept getting the "catalyst efficiency" code after a while, consistently. Then, we'd put a new GM Parts converter on and all was fixed again.