P0420 Toyota - Corrolla S 2004

Question

Check engine was on. when checked (Fault code 0420) Catalyst System Efficiency Below Threshold (Bank 1) appears. needs clarification which parts to be checked properly.

Answer 1

Hi there,

With this code you first check the exhaust system, make sure there aren't any leaks before or right after the catalytic converter. Then you have to check the rear O2 sensor to make sure it's functioning properly. If it is, then the catalytic converter is the problem. If not, then sometimes the rear O2 sensor can be the problem