Even if gas cap was loose and you turned it until it clicked several times, or you filled the tank, it may take a week or so for the check engine like to clear. It's not an instant thing. As for OBD computers, may be $$$$, plus unless you are VERY mechanically inclined and engineering-savy, there isn't a lot worth taking apart when it comes to electronics and computers on the new Volvos, especially on such a new car. Hate to say it, but recommend getting your dealer to check it out if the fault doesn't clear in a few days to a week after the fillup.