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Originally Posted by IRA51
a P0420 is bank 1 I think and it could be anything from an exaust leak to a bad cat . some times a bad O2 can set it . if the down stream O2 does not see much of a reduction it can set it . I would check the O2's first to see if they are good B4 changing the cat .check your coolant level,a leaking head gasket can kill cat's and O2's .
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Correct, p0420 is bank 1 catalyst low efficiency. Outside chance that an o2 sensor will set this code, but is possible. Also needs a pretty good exhaust leak, at least one audibly noticable to set it. But still possible for sure. As said above, check for exhaust leak, o2 scan data would help, but in the end my money would be the cat is bad. In my experience i have never seen a low catalyst efficiency code set that a new converter didnt fix. I even gambled on a couple that had headers installed that were leaking badly and turned out the new cats fixed them.