Q: Code p0420

Hello. P0420 could be due to a variety of faults including even a fuel pump that is not supplying sufficient fuel, causing a lean condition, and hence "fooling" the O2 sensors (pre and post) as to the status (i.e., efficiency) of the catalytic converter.

Codes represent "clues" as to where to begin the diagnosis (not necessarily the parts to be replaced) and thus each potential cause associated with the code (and some causes not even listed in the manual) have to be separately diagnosed, that is the individual components have to be tested.

A certified technician, like one from YourMechanic, can evaluate and diagnose the trouble code for you. Be sure to have them check the fuel pump as I recently had a case of this exact code you are inquiring about on a 2001 Pontiac Montana (3.4 L) and it turned out the fuel pump was malfunctioning and the reported code "fault" (i.e., converter efficiency) was simply a consequence, or "artifact" of that fuel pump failure.