1997 pontiac grand am 2.4l Dear JK:Had miss fire problem first, replacing the ignition parts fixed problem.But, cat went bad.Replaced cat and front and rear o2 sensors.Then gas cap for evap leak.Now have a p0420 code.Is it the cat or rear o2 sensor? or something else? mass airflow or map sensor? Thanks for your help,if you could respond soon it would really help me.Because I think the shop that replaced the cat is trying to get out of warranty. Thanks
JK: Hello and welcome. Since you seem to be in a bind, i will help now. We were all taking a break for a while, its been busy. P0420 is cut and dry 'cat. converter Inefficient' 99% of the time it IS the converter. Have only had 1 bad post cat. o2 sensor cause this in the last 10 years. If the post cat. o2 is bad, it will set a code for 02 sensor, not usually catalytic Inefficient.
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