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PO420 code on 2007 Mercury Milan

Car has 82,000 miles. Replaced battery and one month later emission light came on. Codes PO 305 and 301 cylinder misfires came up. Replaced all 6 spark plugs and 6 ignition coils and intake gasket. Cylinder 5 kept misfiring and burnt out the engine control module so replaced that also. One week later emission light came on again and now have a PO 420 code. Other than replacing catalytic converter is there anything else it could be

Asked by for the 2007 Mercury Milan
i think you need to seek help the misfires poss burn't catt out. the po420 could be a couple of issues.like catt ,o2 senors or exh leaks
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