Typically this code will set when the computer senses that there is a leak in the evap system. What that means is the fuel vapors in the tank are sealed and the system will pull the vapors into the engine to be burned. If the computer senses that the system is no longer sealed, it will set this code. Common causes of this code setting are a loose or missing gas cap or a gas cap that has a bad seal. If the gas cap is good and has not been left loose, the other cause could be a bad evap vent solenoid valve. If the valve sticks open, the same code will set. The vent valve is located under the car near the fuel tank.