Question

2002 Buick Regal GL 3.8L with code P0420 light came on

Answer 1

HI there,

This code stands for catalyst efficiency below threshold. This basically means the bank 1 catalytic converter isn't able to clean the exhaust like it should. Most of the time a new converter is required to resolve the code.

That being said, you can also have an issue with the rear O2 sensor cause this code, as well as a poorly running engine and an exhaust leak.