Catalyst efficiency monitor routines run continuously in the Powertrain Control Module. Catalyst efficiency is determined by some weighted algorithm based on a number of factors over a certain length of time. I am uncertain about specifics, and trigger points are different depending on whether exhaust uses a single CAT or dual CATs or ‘Y’ pipe with single CAT.

But your code is triggered when the O2 switching ‘RATIO’ (in percentage) of the front O2 sensor as compared to the rear O2 sensor exceeds a factory minimum spec (77%) for YOUR exhaust configuration. Your current ratio (at instant of Mode 06 test) was 52%. IN OTHER WORDS, your rear O2 sensor was switching approximately once for every two times the front O2 registered a voltage switch. That equals P0420

I would trust your O2 sensor. It did what it is supposed to. Switched when oxygen level around it changed. If they are bad, they tend to get ‘lazy’ and don’t switch as actively as they should.

Cure any misfires. Check your fuel trims (Short Term and Long Term). Make sure it is not running rich or the CAT is not getting ‘swamping’ with unburned fuel. Otherwise – you probably are looking at replacing the Catalytic Convertor.

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