SES light: Got the code-update!

[PeteG]

About 4 weeks ago, my "service engine soon" light came on. The car was runnining and continues to run smoothly as before, with no noticed performance changes. I thought at first it could be the old "started the car with gas cap not closed" when my wife got some gas in it, but I don't think that's it. It's been 4 weeks and I drive it daily.

I have an appointment at the dealer next Thurs to diagnose, perhaps fix the problem. What things might it be? It's an '01 3.0i w/ about 87k on it. I am the original owner.

Thanks, gang!

Update: Got the fault code read at Autozone (thanks, Boston). It read as an fault of the evaporative fuel system-major. Guy at Autozone said he had a similar problem with his GMC truck. Said the fuel cap itself may be faulty and not creating a proper seal around the filler opening, thus trigerring this fault. He says he can't get me the cap and to stick with OEM.

Does this sound right? Is it a DYI or should I take to the dealer? If dealer, anyone have an idea of cost?

Thanks again, X5World!