PLEASE HELP I VE HAD A PROBLEM WITH O2 SENSOR WHICH WAS SMELLING GAS

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ENGINE LIGHT WAS ON AND IT WAS SMELLING REALLY BAD GAS I VE CHANGED MY O2 SENSOR YESTERDAY AFTER I CHANGED IT IT STARTED AND ENGINE LIGHT GONE .THIS MORNING I WAS GONNA GO MY JOB THE CAR DIDNT START AND IT SMELL LITTLE GAS AGAIN WHAT MIGHT BE THE PROBLEM after i ve changed sensor it worked now it doesnt start ??