Besides a bad gas cap what else would make the engine light stay on. The taurus runs fine as ever and the gas milege seems as good as ever?
About a thousand other things. Have the code(s) read for free at any chain parts store and post them here.
Almost anything.
That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.