At some point in the life of a vehicle, the driver is probably going to notice that the check the engine light is turned on. Most people at this point may give up and take it to a mechanic. Unless your car is in obvious distress, you can take it to an auto parts shop instead and have them run a diagnostics check to pinpoint the problem. While it could be serious engine trouble, it could also be a simple malfunction you can fix yourself. Below are a few different reasons why the light may be on.

1. Gas Cap

One possible source of the light turning on is rather simple. It could be a problem with the gas cap. When the gas cap doesn’t create an air tight seal, it can cause some problems. Your fuel may begin escaping the car via vapor. This can result in poor gas mileage and poor emissions.

Thankfully, the fix for this problem is quite easy. If you do not detect any serious problems in how the car is driving, pull over and check the cap. Simply screwing the cap on tighter will create the seal. Do that and see if the light stays on when you begin driving again. There is also the possibility that there could be a crack on the gas cap. If that is the case, you can buy a new one for a few dollars.

2. Oxygen Sensor

The oxygen sensor is what tells your car’s computer how much fuel you are using. It does this by monitoring the amount of oxygen that remains unburned as it moves through the car’s exhaust. However, sometimes this sensor becomes damaged or corrupted. This could happen due to the sensor getting covered in oil or any of a number of other reasons. If that happens, the sensor will send incorrect data to the car’s computer. This in turn could create a significant dip in what the computer reports as your mileage. Emissions will also suffer.

Not replacing the sensor can lead to serious problems like the catalytic converter failing. Thankfully, replacing one of the four oxygen sensors is rather easy. Just follow the instructions in your owner’s manual. All that is usually required is removing the old sensor from a clip and snapping a new one in its place. A code from the diagnostics test will tell you which one to replace.

3. Catalytic Converter

The catalytic converter is the part of your car that does the job of cleaning its emissions so they are less harmful to the environment. It takes carbon monoxide and converts it into less harmful gasses. Having a faulty catalytic converter can cause serious problems. Not only will your emissions be much dirtier, but you won’t be able to accelerate anymore either.

If you are performing proper maintenance, then your catalytic converter shouldn’t fail. Faulty spark plugs and oxygen sensors are a far more likely culprit. Still, the catalytic converter may need to be replaced under certain circumstances. If it isn’t fixed, the entire car could fail. Unfortunately, you will need to pay a professional mechanic about $2,000 to replace it.

4. Mass Airflow Sensor

Another sensor producing errors can also cause the check the engine light to turn on. It’s the sensor for mass airflow. This sensor checks the amount of air that flows through the engine. Based on that information, it then tells the computer system how much gas to add. If it’s broken, it could lead to higher mileage, poor emissions and even the engine stalling.

Most often, this sensor breaks because the air filter was not replaced like it should have been. Replacing the sensor in an auto shop could cost over $200. While you can probably do it on your own, it might be a good idea to let a mechanic do it to see if there isn’t another serious problem causing the issue.

5. Spark Plugs

Lastly, the problem could originate with the spark plugs. As you may already know, the spark plugs are used to create combustion in the engine. When the spark plugs become faulty, you’ll experience a misfire. This will cause you to feel a jolt in the car when accelerating.

Unfortunately, spark plug failure is part of owning a car. You can’t really do anything to prevent it. With newer models, you will have to replace the spark plugs once every 100,000 miles. If it turns out the check the engine light was switched on due to the spark plugs, you should just replace them.