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I am sorry your having this problem
this has a brand new GM converter or a direct fit aftermarket one and it still sets a 420 code ?
was it a direct fit or was it welded in place ?
this code is set one of 2 ways
1 ) the cat converter is not up to original specs and is not converting correctly still setting the code
2 ) either the front or rear O2 sensor is reading wrong triggering the 420 code
what sets this code is when the front O2 sensor just about matches the rear O2 sensor as if they match reading or are very close the computer sees that and sets the code as a good working converter the rear O2 reading should be a lot lower than the front O2 sensor showing the converter is converting correctly
at this point replace the rear O2 sensor if it still sets the code the converter is not up to standards to clean the exhaust correctly
# ***** in this picture located under the car
the step would be to install a GM direct fit converter
very simple to do
pick up as new one and get under the car and it is very visual to change
the welded into place aftermarket cat converters where poor quality and 50 % of time will still set 420 codes as they do not meet the GM standards
yes it should be under there warranty