P0420 Explained: Do Not Buy a Catalytic Converter Until You Watch This

About this video

P0420 Catalyst System Efficiency Below Threshold Bank 1. Every shop quotes a catalytic converter, and that repair runs anywhere from 800 to over 3000 dollars depending on the vehicle. In this video our ASE Master Technician with over 30 years of experience explains what P0420 actually measures, why a good converter often sets this code, and how to test properly before spending that money.

The code does not say your catalytic converter is bad. It says the computer compared the upstream and downstream oxygen sensors and the downstream sensor is mirroring the upstream one instead of staying steady. Plenty of things cause that besides a failed converter.

WHAT P0420 ACTUALLY MEANS:
Your engine runs two oxygen sensors around the catalytic converter. The upstream sensor swings up and down as the engine cycles rich and lean. The downstream sensor should read a fairly flat steady line, because a working converter stores and releases oxygen and smooths the swings out. When the downstream sensor starts copying the upstream waveform, the computer concludes the converter is no longer doing its job and sets P0420 for bank 1 or P0430 for bank 2.

WHAT CAUSES P0420 BESIDES A BAD CATALYTIC CONVERTER:
Failing or lazy downstream oxygen sensor
Failing upstream oxygen sensor giving false readings
Exhaust leak ahead of or between the sensors
Engine misfire dumping raw fuel into the converter
Running rich from a leaking fuel injector or bad fuel pressure regulator
Running lean from a vacuum leak or dirty MAF sensor
Burning oil or coolant, which coats and poisons the converter
Bad coolant temperature sensor keeping the engine in open loop
Aftermarket or universal converter that does not meet OEM specification
Recently replaced converter that was never given a proper drive cycle
Wrong or contaminated fuel

WHY THIS CODE COMES BACK AFTER A NEW CONVERTER:
If you replace the converter without fixing the underlying cause, the new one gets destroyed the same way the old one did. A misfire, an oil burning engine or a rich condition will kill a brand new converter within months. Fix the cause first, then the converter.

SYMPTOMS THAT COME WITH P0420:
Check engine light on, usually steady
Failed emissions or state inspection
Rotten egg or sulfur smell from the exhaust
Slight loss of power or