When the Check Engine light on a car is illuminated, it indicates the onboard diagnostic computer has detected a problem with the vehicle's emission system. The problem is due to a variety of conditions, ranging from a loose gas cap to serious engine problems.
The first response of the driver to a Check Engine light should be to drive to a safe spot, and tighten the gas cap. As of 2014, the design of most gas caps is to click and turn freely once the cap is sufficiently tight. Turning it clockwise so that it clicks three or four more times seals the opening and often clears the problem. If it remains illuminated, a mechanic should use a scan tool to read the diagnostic code.
The scan tool returns a diagnostic code that tells the mechanic which diagnostic procedure is appropriate for determining the underlying cause of the illuminated Check Engine light. The mechanic should perform the procedure in the exact order indicated, and ensure the readings such as temperature or voltage are exact matches with those the vehicle manufacturer provides. If the mechanic performs the tests out of sequence, the results are sometimes meaningless.
The Check Engine light differs from the Service Interval light. The latter illuminates after the vehicle passes a certain number of miles, to remind the owner that certain services are due for the vehicle.