It could be a catalytic converter. However, it could be an O2 sensor or other item. The Envoy seems to idle poorly in park, with the RPMs moving from 300-800. It is like it can't determine where to idle. It seems worse with the air conditioning running in hot weather. I had a cylinder 4 misfire that I fixed by replacing the boot/coil, but didn't get the P0420 until after that was fixed.
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2002 GMC Envoy Question: What could a P0420 code mean? What steps should I follow to correctly diagnose?
A MISFIRE CAN DESTROY A CATT IN NO TIME THAT MAY BE YOUR ISSUE,SEEK A PROS HELP AS TO NOT WASTE TIME AND MONEY