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P0420 Code on 2005 Toyota Corolla

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Can a P0420 Error Code show up when a ECM is defective? What the code points toward is the Catalytic Convertor being bad? Can the Cat actually be good, it's just the ECM defective, and throwing the code in error?
(1) Answer
Anything can happen when an ECM fails, including generating false codes.
P0420 through P0429 are catalytic converter codes. Codes that refer to the catalytic converter don't always mean that the converter is bad, they are diagnostic clues, not always the answer in themselves. Diagnosis by an experienced person, using codes and data are the way to diagnose catalyst questions.
If you want to find a specialist, here's a directory link for you:
https://repairpal.com/directory?address=02038&car_brand_names=Toyota