The check engine light (aka the malfunction indicator light) normally lights up as you start your car and then promptly turns off once it has settled down. If it refuses to go off then it means that something’s wrong with your car. The problem, however, is the check engine light is awfully vague about what’s wrong. Some of the factors that could lead to it being left on include:
A faulty oxygen sensor is not providing accurate data to the onboard computer and could mean lower mileage. This is one of the most common causes for check engine lights, but ignoring this problem could break the catalytic converter of your car – a component that can cost around $2,000 or more to repair.
A loose gas cap that leaks fuel vapors which in turn leads to lower mileage and more emissions than normal. This problem can be easily fixed by buying a new gas cap for less than $10, but ignoring the problem could lead to higher gas expenses over time.
Degrading mass airflow sensor that if left unchecked will result in stalling, increased emissions and lower mileage. The sensor is not that expensive to replace since it is easy to install, but ignoring it will not only increase gas expenses but can lead to accidents if you stall at an inopportune moment.
Failing spark plugs that will cause your car to randomly bolt forward while driving. They’re cheap to replace and is definitely worth it considering the risk of suddenly accelerating while on the road.