99 hyundai accent check engine light comes on after rain

I have a 99 hyundai accent GL 4door automatic transmission...whenever
we get a steady rain or hard rain for a day or more the next time i
use my car the check engine light comes on shortly after i will be
driving down the road..at first it will flash just a few times and
then will stay steady...it might be like this for several days
depending on the weather or how much i drive the vehicle but i know it
won’t go out until something is completely dry it seems..maybe a
couple of dry weather days etc...then it might not come on for weeks
or months..the car has 136000 miles and i have owned it since july 04
when it had about 65000 miles..i have put a new timing belt a few
miles after i bought it..the car for the most part runs good except
when this happens..even though it has been doing this for several
years now i have never taken it to a garage because either i did not
want to pay a garages diagnostic fee or when i wanted to stop in the
check engine light was not on...now its spring time and we usually get
alot of rain and would finally like to find out why this light comes
on and why it sputters at first when the check engine light is
flashing....does anyone know how to get the codes and what they
actually mean?i have read on some of these sites people say places
like autozone or advance autoparts will do a free test for the check
engine light...is that true?i hope i can get some feedback on here.

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