CODE 0420 ON A 2002 MONTE CARLO SS 3.8L ALSO RESONATOR PIPE IS LEAKING WILL THAT CAUSE THE CODE
"catalyst system efficiency" yes, that's you buddy. Get it repaired!
same hear, get leak fixed, causing oxygen sensor to read wrong and causing computer to set the code.
Hey @TIMFLYF! I was wondering if fixing the resonator cleared the P0420. Many people say that a leak downstream of the Cat will not cause a code. I have feeling it might. I'm facing the same situation you were facing, so I'd appreciate your feedback!
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