On most cars, the check engine light is directly related to a "fault" in the emission control system somewhere, and could be anything from a loose gas cap to a bad oxygen sensor, etc. If something is out of whack in that system check even for only a moment, the computer throws ups it hands and screams " wait a minute, something is out of the ordinary here...check engine light on. If it is a temporary glitch, the light will often go out on it's own if that particular part of the system checks out during the next "cycle", which usually consists of a preprogramed look by the computer at varied driving conditions over a set number of miles. I had a Volvo that was finicky about gasoline. If I ran Citgo gas from a certain station for many fill ups in a row, check engine light would come on. Change gas brand and station for a few tanks and it would go off. So it appeared the cause in that car was "bad gas". I don't know if it was contaminated with water or something, but whatever it was the emission control system in that car didn't like it. Since then, I try to fill up with "top tier" gas only, when I can...usually Shell regular for my HAH. If I had a Conoco nearby, I would switch to that.