The most common reason that the check engine light comes on is because there is a problem with the emission control system. This can be caused by a number of things, such as a faulty oxygen sensor, a problem with the catalytic converter, or an exhaust leak. Sometimes, the problem can be something as simple as a loose gas cap. If you notice that your check engine light is on, it's best to take your car to a mechanic and have it checked out so that the problem can be fixed before it causes any further damage to your vehicle.