Although the exact causes vary depending on the type of vehicle, the most common cause of the check engine light coming on is a faulty oxygen sensor. A loose gas cap also causes the check engine light to turn on, so owners should check this first.
Many owners confuse the check engine light with the service light, according to Edmunds.com. The lights may look similar and be in a similar area, but the service light simply means that the vehicle is due for an oil change or other routine maintenance. The check engine light generally comes on if there is a problem with the emissions system in the vehicle.
Even if the gas cap is on tightly, it may still be the cause of the problem if the seal is damaged or the cap is otherwise faulty. A faulty catalytic converter is another common cause of the check engine light. Vehicles with broken catalytic converters generally cannot pass emissions tests, have poor fuel efficiency and may be at a higher risk of overheating.
A faulty mass air flow sensor also causes the light to come on in most vehicles. Replacing this is important because it can cause additional damage to the catalytic converter, oxygen sensor and spark plugs. Worn, damaged or missing spark plugs and wires also often cause the light to turn on when they need replacement.