If the cas cap is not tight or if the gasket is leaking so that it won't hold pressure the check engine light will come on. The code will be something like "evaporator emision failure" or something similer. Make sure you check you cap for damage, and tighten till it clicks after gassing up. The fuel tanks run with positive pressure of about 3 lbs and if not the light will come on. I would have someone reset the code, make sure of the cap, and see it it comes back on. You also may have a leak in the system, loose fill hose or, whatever. Good luck.