what does service engine light soon mean? on 2001 Ford Expedition

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2 answers
your pcm has picked up a code from a sensor that is seeing abnormal readings have it scanned and see what code you have. auto zone will do that for free. the code only means abnormal readings but it could be the circuit,the sensor or related components so don't just start throwing parts at it
try a new gas cap and make sure it is on tight $8-$10 for new