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From what you are describing here, it does sound like the gas cap will need to be replaced. When the Check Engine Light comes on and points to a "leak in the EVAP System" then followed by a "Gas cap" message, very commonly this is a faulty gas cap. Check the underside of the fuel cap for any cracks or signs of defects that may keep the cap from sealing correctly. If there are any doubts about the inspection of the cap, replace it. Keep in mind, you will need a cap that meets OE Specs. Those "universal" or "locking" fuel caps sold at after market parts stores do not meet OE specs, and will not seal the system correctly. If the Gas Cap message returns, and with a Check Engine Light soon there after, then the problem will be somewhere else in the EVAP (emissions control/recovery) System. The system will need to be "smoked". This is where a machine forces smoke through the system and any leaks can then easily be found. If the smoke test shows no leaks, then EVAP System components will need to be tested based on the "p-code" that is present. When/If the Check Engine Light returns , you can have these p-codes read FREE (except in California) at any local "big chain" part store (ie. AutoZone, PepBoys, Advanced, etc). If you found this answer helpful, please help me and leave a rating using the "stars" at the top of the page or click 'accept'.Rating or accepting my answer does not cost you extra. It allows me to be credited for this Q&A.