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!!!