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[childers.paula AT epamail.epa.gov]

the "gas cap loose" thing happened to our car last year. amazingly, it
is legitimate. i don't remember exactly, but it was something about the
fuel/air mixture in the pressurized system (almost all newer cars have a
pressurized fuel system) was off.   for us, it was not a loose gas cap,
but an overpressurization after we allowed the tank to get v-e-r-y empty
and then filled it to the brim on a hot day.  check your owner's manual,
as it may have a recommendation for how to resolve the issue without
diagnostic/$$$.  our manual had step-by-step instructions for getting
the light to go back out.  i would imagine a web search would turn up
similar stuff.   of course, if you follow those instructions and the
light is still on, and then you try that battery trick and the light is
still on, then you gotta problem.