1st, make sure you put your gas cap on tight enough, that can cause codes for emissions system. Next you may have to check the vapor cannister and vent valve, along with the purge control solenoid. Those are the major components of the fuel tank vent system. Check the 2 purge valves with a continuity tester to make sure they work, also make sure there aren't any kinked or torn lines to/from the cannister. The tank is supposed to keep a certain pressure inside it, if there is too much (no vapor is venting) or too little (vapor vents constantly with no control) you will get a light.