Your check engine light may be on for a variety of reasons, but fear not; while some problems related to this light are as serious as you may expect, not all require expensive repairs.
1. Loose Gas Cap. Hopefully, this is the reason for your problem—and yes, it is actually one of the most likely issues. Sometimes, you simply didn’t screw the gas cap on tight enough the last time you filled up, or the cap is cracked and must be replaced.

2. Failing Oxygen Sensor. If the oxygen system is possibly causing a fuel economy decline, one of our trained technicians can confirm using a diagnostic code reader.

3. Faulty Spark Plugs. You may require new spark plugs if they are malfunctioning; ignoring this issue may keep the car from starting altogether.

4. Faltering Catalytic Converter. If you ignore spark plug failings or the oxygen sensor issue mentioned above, the problem may grow and your catalytic converter may fail altogether. Needless to say, this will result in a check engine light.