Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
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This code almost always means the converter has failed and if you are burning coolant, oil, or too much fuel then it can cause converter to fail early. A OEM converter should also be used to prevent converter failing right away. The oxygen sensor could be the cause but not common and there is no TSB. Let me know if you have questions, thanks.
P0420 Catalyst System Bank 1 Low Efficiency - With Automatic Transmission