HELP Check Engine P0420
 

I have a 2015 Buick Encore that keeps throwing code P0420.I have replaced all of the following items...Catalytic Converter, flexpipe, intake manifold, valve cover, pcv, both bank 1 and 2 sensors. The light goes off and after driving for about half hour to hour light comes back on form same code...car runs great which is driving me crazy because of the engine light..any suggestions...