I have cat code PO 420 on my 99 accord EX, 4cyl. I have a few questions. I had new pipes and muffler braised on. The muffler shop said the cat looked good. Should I start with trying a new O2 sensor? Could it have heat damage? Also, I need to know if I have 2, O2 sensors and do I change both? I think this is my "family" 3PL4 on the hood, F23A4 on my motor.