Tank Filler Pipe And Cap problems of the 2005 Mazda MAZDA3

Check engine light on for the fourth time since I bought the vehicle. Problem found by us, was gas cap loosened up and was just sitting in there, when we walked to the back of the vehicle to get packages out of the truck, smelled gas. Have had this vehicle to the dealer every time the check engine light came on and the one time they told me the gas cap was loose and I insisted I put it on tight. Of course, I am a female and they try to make you look like an idiot, so I didn't believe them, thought they were just saying that was the problem and resetting the code, so imagine my surprise when we smelled the gas and checked the cap and it was loose. We had already run half a tank of gas out of the vehicle before the check engine light came on which is approximately 200 miles since fill-up. When the check engine light comes on, it is always for the same code, which is an emissions failure.