The "Check Engine" light has been on on my 1990 Hyundai Excel GL (that's not a typo, I bought it new in October of the beginning of the last decade of the last century :O)) for several years.
Hope you've stopped laughing by now.. but it's true, and the dealer can't get the computer chip code to figure out why it won't go out. The car runs great and I've kept up with all the recommended service and various repairs here and there (I won't list them, but a valve job was The Biggy last year and the Second Biggy at about 5 years old was the fuel pump) and the new Midas muffler every 18 months or so (free on Lifetime Warranty), etc.
I've had various mechanics scratch their heads and say, "Well, if it's running right and the light won't go out maybe you should just disconnect the lamp in there."
It's a minor annoyance given the car's so good to me. But I like tinitis in my ears to be the only thing I can't do anything about as far as annoyances go. :O)
Any ideas?
-- Just Joe