Catalyst system efficiency below threshold (bank 1)
The downstream O2 sensor is tracking the upstream one too closely, which tells the ECM the converter is no longer storing and releasing oxygen the way a healthy one does. It does not automatically mean the converter is bad.
The post-catalyst O2 signal mirrors the pre-catalyst signal beyond the calibrated switching ratio over a complete monitor cycle.
Generic SAE definition. Manufacturers extend and sometimes override these, so the enhanced code on the vehicle in front of you may be narrower than what is written here.
This is the list every other P0420 page will give you. Read it, then read the part underneath it.
Every one of those 5 causes is now a decision somebody in the bay has to make, under a clock, on a car a customer wants back today.
We could mock one up. We’re not going to. A fabricated example of our own product’s output is exactly the kind of thing this page is criticizing. Verdicts appear here as the pilot produces them on real vehicles, with the evidence they actually stood on.
The pilot runs on your existing scan tool and your live repair orders. Bring us a car that's throwing this code.