re: Check Engine Light after Filling-Up

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That code could be hundreds of different things on most vehicles

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P0420 is the same around the board not hundreds of things…

P0420 is catalyst low Efficacy bank 1

This is your catalytic converter

That is 1 code that 99.9% of the time takes no additional diagnostic your converter is clogged up and causing excessive back pressure into the engine tossing that code. Only one way to fix it and that’s to replace your catalytic converter