2001 Cadillac DeVille DTS Q&A

2001 Cadillac DeVille DTS Question: had an emission test done and failed with a P0420 error code. help plez

I am fairly knowledgeable about automobiles. anyway, wondering how you would stack up the most likely causes of this error code that I think comes and goes and seems to occur when I put the pedal to the metal. The error is easily resettable by me and stays away for weeks or until I punch my caddy. -
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