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When the Check Gas Cap light comes on, this can also mean there is a problem in the EVAP system (emissions control). When this is the case, the Check Engine light comes on shortly after, and the "p-code" (diagnostic trouble code) is stored in the on-board computer. This very well may be a faulty fuel cap, but, sometimes, it can be a bi tmore complicated. The best bet at this point is to have the "p-codes" read from the on-board computer. Most local "big chain" parts stores (like AutoZone) will read these codes for Free (except in California). Once you have these p-codes, you can more accurately and efficiently diagnose the actual cause of the problem, and then make the repair. If you get a code of P0455 or P0442, first step is to replace the fuel cap, and check the fuel filler neck for any signs of damage that would keep the cap from sealing tightly. Any other code will require additional diagnostics.