Pressure release from gas tank when removing cap

SOURCE: Check Engine Light

There are many many problems that can cause your check engine lite to come on. Some of those problems will not let the lite go out until it is cleared with a handheld computer. (Even if the condition that caused the lite to come in, has cleared) But as far as the 2 comments go, a fuel injector cleaning would be a good invsement here. It is a good possibility that this is your problem and if it isn't, it is still very good for your engine performance and fuel consumption so wouldn't be a bad investment. If your vehicle is still under warrenty I would also talk to dealer about reading computer. Even if the check engine lite is not on, some codes will store in the computer that it came in and cleared so the dealer would be able to tell you what code came in to cause the lite to be on. It would at least be a starting point to look at for troubleshooting.

SOURCE: Check engine light/gas cap will not stay tight.

Yes, the gas cap will cause the Check Engine Light to come on. Will set code P0441 if I remember correctly. It should be okay to drive it. If you would like to confirm the code, take the truck to an auto parts store like Autozone and have them hook up their scan tool and read the code. If its P0441 it will probably be something like EVAP or Evaporative system leak.

Its an easy enough fix while your there too, just pick up a new gas cap or you could pick up an OEM one from the dealership.

SOURCE: Tighten Gas Cap Error Message

seems that your vent hose on the tank is plugged so the tank is over pressurrizing sending the error code and not allowing you to fill it all the way unclog the vent tube and this will solve the problem

SOURCE: check engine light on says check gas cap

The evaporative emissions system uses a vaccuum pump to check the fuel system to make sure that fuel vapors cannot escape into the atmosphere. If you leave fuel cap loose the computer will detect it and set a computer code and turn on the check engine light. If the cap is tight and the light still comes on, you may have other problems. Hopefully your vehicle is still under warranty, as evaporative emission system leaks can be very difficult to locate and repair, even for the dealership mechanics.

SOURCE: 06 chevy impala v6 3.9 liter engine. Check engine

there is a large evaporative leak. keep away form ottozoo, they dont have the brain power to actually diagnose a light bulb., their job is to sell parts., if they sell you the wrong parts tough.Take it to a qualified repair shop that does a lot of GM work or the dealer.