Check gas cap

I have an 04 GT and my check gas cap keeps turning on whenever my car starts, I have replaced the cap thinking that was the case but it still comes on and now the check engine light comes on as well.

Maybe you've got a bad gas cap, can you see any cracks in it? A check engine light can be thrown because of a loose gas cap as well...or a bad one.

also just as a side note when you tighten the cap on you should always have it click 3-5 times that way you know it is on all the way.

my mother had an issue like that once ans that is what she got told (and she got the cap replaced) ever since it hasn't had that porb, not sure which it was the new cap or the tighten or even both for that matter

I bought my 04 used and had that problem immediately off the lot. At first the dealer service writer blamed it on the salesman who gassed it up before delivery with the engine running. He reset the computer the light went away...for 12 hours. It happened again. I still had warranty left so they replaced the cap. But I believe the computer has to be reset after it happens. By the way, my cap looked good, but the seal was defective. Also, if you take the cap off with the engine running, you will get the same message, and I believe it has to be reset. Not sure how to reset the computer since mine was under warranty and handled by the dealer service dept.

If it's just a Service engine light, then disconnecting the battery for a half hour will do it.