The P0420 DTC is generated by the rear (downstream) HO2 Sensor when it
senses too much leftover oxygen after the catalytic converter does its job.
This causes the sensor to "switch" high-low just like the upstream HO2
sensor. Sometimes there may be a misfire or a lean condition like a manifold
leak which causes this DTC. More often we find the catalyst is just worn,
coated or deteriorated from long use and having had the above-mentioned
upstream defects at one or more times in its life. That's one good reason
why it is so important to promptly fix any misfires, rich or lean
If you are satisfied the upstream problems are repaired, it's time to
replace the cat.
Chino Autotech Inc.