One of the most common reasons a check engine light comes on is a poorly secured gas cap. Gas caps should be twisted at least two full turns after they begin clicking.
By checking a vehicle's gas cap to ensure it's tightly fastened, a driver can save herself costly trips to a mechanic by fixing the problem herself.
Another common issue that causes the check engine light to come on is a burned-out oxygen sensor. This device ensures a proper fuel-to-air mixture in the vehicle's engine, and a faulty sensor can cause performance problems that may, at first, be undetectable by the driver. Taking the vehicle to a mechanic can help determine if this is the problem or if there is another underlining issue at hand.
The third most common issue that triggers the check engine light is a catalytic converter that is malfunctioning. This can be a rather expensive repair, but if it is left unaddressed, it can eventually lead to a total vehicle break-down. Because of the expense, taking the vehicle to a trusted mechanic is a wise move, as is getting a second opinion.
Finally, the fourth most common reason is a malfunctioning mass airflow sensor. This device, similar to an oxygen sensor, measures the amount of air flowing into the engine. If it is malfunctioning, similar problems to those that result from a malfunctioning oxygen sensor can arise, including lower fuel efficiency and reduced power.