ASE Master Tech Explains P0420 Catalyst Efficiency Below Threshold

About this video

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