It sounds like you have an exhaust leak, or the engine is running poorly, how does the engine run? If okay and the cats and O2 sensor have been replaced the only other thing, is it can be a bad PCM because it has lost the ability to monitor the emission system. Here is the location of the PCM and how to change it out in the images below. You can get a preprogrammed unit by searching Google or Ebay.
Here is generic info on the p0420 code:
Faulty catalytic converter: The catalytic converter may be worn out or damaged, resulting in reduced efficiency.
Check out the images (below). Please let us know what happens.
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Thursday, May 25th, 2023 AT 9:56 AM