the check engine light will stay on for awhile after you correct the gas cap problem. just keep driving it for about a week or so, if it doesnt go off run by somewhere with a decent scan tool and have them check the code. my gf has a bad habit of this but luckily i have access to a 4k$ motis scan tool at work lol. also something to keep in mind, a modification does not void the warranty of the car, only the part you modded. now when you get into internal engine they you have a fight on your hands, but nothing you've done will have voided a warranty, i generally work hand in hand with some dealerships and have made some friends in the parts and service areas. so no worries give it about a week or so and see if the light goes off, or just run it by a parts place with a scan tool.