Did you know that your check engine light can be turned on by something as simple as a loose gas cap? The vapors above the liquid fuel in your gas tank are one of the largest volumes of hydrocarbons in your car. The EVAP system is responsible for managing these vapors. Air entering and exiting the fuel system is run through a series of filters to keep the vapors from escaping. While the car is running and the tank is between 1/3 and 2/3 of a tank, the system is tested to see if it can hold pressure. If a leak is detected the system turns on the check engine light, with a code indicating a leak in the EVAP system. It is up to the technician to find the leak in this complex system of filters and hoses. However, if the gas cap is left loose or the car is run without a gas cap, the EVAP system will see this as a leak and turn the light on.

Because it is an easy thing to do, when your check engine light comes on, check your gas cap. Did you drive off and forget it, then find it later? It may be that easy to fix.