The "Check Engine" or "Service Engine Soon" light comes on the dash when your car's computer detects a problem that affects your vehicle emissions. It could be a problem with the engine, transmission, one of the emission control systems or as simple as your gas cap is not on tight. Think of it as an early warning that may help you catch a problem with your car before it gets worse.

Here are the five most common reasons your "check engine" light might come on.

1. Faulty oxygen sensor
The sensor measures the amount of unburned oxygen in the exhaust and tells the car's computer how much fuel is in the tank. If a faulty one is not repaired, the car's gas mileage could drop, since the sensor is sending incorrect information to the car.

2. Loose or missing gas cap
Technicians will often tighten the gas cap for free, or replace it for a few dollars. If it's not replaced, gas will evaporate from the car and decrease its gas mileage.

3. Broken catalytic converter
The catalytic converter uses a catalyst  most often a precious metal such as platinum to convert harmful gases left over from combustion to less harmful emissions.

4. Malfunctioning mass air flow sensor
This sensor measures the amount of air supplied to the engine, which determines how much fuel should be delivered. When it malfunctions, it can result in a loss of power to the car, surges during acceleration and a decrease in fuel economy.

5. Misfiring spark plugs
Spark plugs are small but essential, since they make the car go by ign