This page is meant to be a reference to understand common diagnostic trouble codes that can cause your check engine light to come on and what things can cause each code. If the suggestions we have don't help, its best to take your car to an experienced performance shop for further diagnosing.
P0420 - Catalyst System Efficiency Below Threshold
This code means your catalytic converter is not performing up to the ECU's required level. If your down-pipe or J-pipe is stock, then it is likely it is simply worn out if your car has higher miles. If you have an aftermarket catalytic converter, it might not be good enough to satisfy the ECU and might need to be replaced with something better. Typically GESI catalytic converters can and will satisfy the ECU and keep this code away so we recommend only using aftermarket systems that use these.