Service Engine Soon Light stays on. Tightening gas cap worked at first.

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As mentioned the service engine soon light came on and stayed on. Opening and re closing the gas cap seemed to work at first, but the light came back on after a few days and stays on. The vehicle has high mileage,it does use a small amount of oil although not much, no leaks. Also add coolant to overflow about once a year. Vehicle has never over heated, oil changed regularly and runs fine.