I fixed my 290K miles accord P0420 by Amazon “Cataclean 120019 Cataclean - The Original Liquid Science - Complete Engine, Fuel & Exhaust System Cleaner”. Pay attention to when O2S1 ramp up and O2S1 goes down on OBD2 scanner, it means ECM puts more fuel but O2S2 still gets more O2, it means catalytic converter doesn’t use the O2 in its ... More @Wikipedia
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