what part needs to be replaced code p420

Question

erase code it keeps coming back on

Answer 1

P0420 Catalytic efficiency

The best way to approach this to check this is to check that the rear 02 sensor is properly reporting the condition, The best way to do this is with a scan tool that can do graphing,

​ Here a Dodge which set a P0420, The rear 02 sensor was checked what was found was a faulty rear 02 sensor that did not report the correct condition, A rear 02 sensor was the fix,