On my 01 Park Avenue the Service Engine Soon light came on. Tested it and code P0420 Catalyst Effiency Low. My mechanic cleared the code it lasted for a little while because he said he has had a few GM'* that have popped up with this code when nothing was wrong. It came back about 2 weeks later. His car is due for smog next year. He wanted to know if the code is cleared before the test will the car still fail? I think it will but I don't want to look like an idiot.

It may be that your rear O2 is on the fritz. Try changing that out first, then see if the problem returns.

already changed that when the problem started.

Mind me asking what brand of )2 you put in it?

This could also mean your cat is dead. And I am pretty sure the federal mandated warranty on the converter is past the time and mileage limit.

If you chose an aftermarket, be forewarned there can be out of the box issues!! More common place than most believe; I would stick with AC Delco/GM if at all possible.

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