Does your test pipe have a bung for the O2 sensor? If so, the reason the light is on, is that the O2 sensor is getting a reading that it's not supposed to get. A way around this, is to tie the sensor up under the body, and wrap it in tin foil...just expose it to atmospheric air.
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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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