Gas cap is always loose

Look closely at the metal tabs that the cap threads onto... I suspect you might see something that's a little bit bent that can be bent back into place with some pliers. If it's hard to figure out try to find someone with the same car and compare the difference.

If all else fails, you could buy one of these and you, a friend or family member can learn how to replace it and we can help walk you through it:

 

You know the generation 2 has two different cap types You can't mix them up One has two ears on it and one threads which do you have when you look at the tank hole behind your door there are two different types You might want to assess that out first. I have both types here I think the '09 is a screw in and the '07 has the tabs I would have to go physically look mine are original caps still have the strap that keeps them attached to the door and all that

 

Also try a layer of Vaseline or similar lube on the o ring. My wifes Mercedes has the CEL everytime we go back to Colorado and the altitude changes. More lube always helps I have found.

 

By just looking in the silver gold metal area with the fuel tank door open you can tell whether you have the two ears or the threaded cap well at least I can if I just look in the door.