What converter did you replace it with? This is where quality really matters. Most cheaper aftermarket cats will not correct the 420 code or do so for only a short while. I'm not saying that is your issue, just my own observation.
http://www.obd-codes.com/p0420
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Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.
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