What are reasons P0420 can come on for nissan altima 2001

Question

I just had my car fixed for code P1441 and the selenoid valve was replaced -not the canister

I went to auto zone andthey listed three or 4 possibilities -
faulty fuel system
ignition starter
catalytic converter
vacuum hose leak
large evap leak -

Before it was small evap leak
How do I begin to diaganose or the mechanic?

Answer 1

HI there,

The most common causes for this code would be either a bad catalytic converter, a poorly running or misfiring engine or a bad rear O2 sensor. Diagnosing requires a scan tool that can read the live data that the O2 sensors are seeing, then depending on their input it can be determined if the converter is the actual problem or not.