The gas cap being loose has a code that is all its own. That code you have has to do with the catalytic converter, not the gas cap. They are very expensive from the dealer. You can try to find a used one, or you can buy an aftermarket high flow cat. I would reset the ECU and see if the code comes on again.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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