The reason they said there MIGHT be a charge is because most stealerships have a 1 hr min charge for service. Theydon't know ifit's underwarranty or not, until they know EXACTLY what is wrong with your car (by a code scan and a visual inspection). They will write you up a service ticket, andthen take the car into check it. The CEL came on because a part is possibly faulty. If the said part is under warranty, or related to a TSB or recall, thenthey will replace it for free, and not charge for the diagnostics. That is why they can't say it's underwarranty until they have completed an inspection. I would not be worried about having to pay anything, because I'm 99.9% sure thatyou won't have to.

The inspection at the stealership will still have to be done even if you have already went to AutoZone,and know what code is causing the CEL (although you can tell them the code when you make the appointment, and they might be able to tell you if it's covered under warranty or not).