the code 0420 catalyst system efficiency below threshold (bank 1) keeps coming on
You need a new catalytic converter. My '02 Impala threw the same code. There were so many failures on the '01 & '02 Impala's catalytic converters that Chevy actually extended the warranty to 10 years or 120,000 miles, whichever came first. As your driving the PCM (computer) tests the catalytic converter by momentarily enriching the air/fuel mixture. This will increase your emissions level unless the catalytic converter absorbs the excess emissions. If the catalytic converter can't, the PCM (computer) picks up on it via the downstream oxygen sensor & sets the trouble code. HTH. -Jim
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