Question

2002 Toyota Celica with codes P0420 and P0171, what do the codes mean?

Answer 1

The P0420 code is a bad catalytic converter code. This means the converter is not working efficiantly enough to make the rear O2 sensor happy. In most cases a new converter is in order.

The P0171 code is a lean mixture code on bank 1. There are several causes for ythis type of code. Most common cause is a vacuum leak, low fuel pressure or a possible mass air flow issues.