Check engine light comes on and off

The check engine light illuminating indicates there is some kind of problem being noted by the engine's processor. This problem affects engine performance and / or vehicle emissions controls, and as a result must be serviced and repaired. Diagnostic testing is required to find out what information has been stored related to whatever failure is occurring. Then this data needs to be used to aid in troubleshooting steps needed to drill down to the cause of concern. Good luck.