Has it got a gas cap

The "gas cap light" actually does not mean the gas cap is loose or defective.It is telling you there is a leak in the fuel system somewhere,usually a vapor line or stuck or defective valve.Not very often is it the gas cap that causes the light to come on.Time for a scan and a smoke test to fine the leak.

First, you have to have the trouble codes cleared from the computer memory to make the warning lights go off.
And the problem may be a leak in the gas tank venting system instead of the gas cap. The gas cap is the most often cause, but not always. You may have to get a shop to check for leaks.

Can't dismiss or second guess what you were told

Auto Parts Store counter people are not mechanics
or techs

Check for codes,then diagnose

Test,test,don't ask questions

CHECK GAS CAP COULD BE LOOSE OR MISSING OR LEAKING CAUSING EVAP LEAK IN WHICH WILL CAUSE CHECK ENGINE LIGHT FLASH ON..

One of the problems that trigger a check engine light is either a faulty gas cap, gas cap not on all the way or no gas cap. Once you replace the gas cap you should be able to reset your check engine light or it may automatically adjust itself.

The check gas cap light was in response to complaints of the check engine light being on for loose/missing gas caps. So now when the PCM sees a large EVAP leak the first thing it does is illuminate the check gas cap light (on models equipped with the light). Since it doesn't know if the gas cap is on or not it can have other issues like a vent solenoid stuck open which the PCM would be unable to distinguish between that and the gas cap left off.

There will still be a trouble code in the PCM for the EVAP system.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

You have most likely obtained the wrong type of gas cap for your F150. Try using a similar gas cap from another vehicle you may own, or one of your friends. You can also try using the new gas cap on the other car to see if a similar problem occurs.

By "Replace the gas cap", do you mean put it back on, or buy another one? If you fuel these trucks with the engine running, or don't tighten the gas cap, the light will come on. It will also come on when the seal in your gas cap goes bad. I have had to replace the gas cap on my jeep and dodge already. It fixed the problem both times.

put a new gas cap, those system are pres. even if
the cap look ok it maby leaking replace with a new
one it should solve your prob.
eng.ligh come on because of the gas cap

have you replaced the gas cap? most of the time when a check engine light is on for the gas cap, its because the cap isnt venting the fumes properly or the cap isnt on tight enough.