Stuck flapper indicates nothing more than the presence of corrosion on pre 2005 Mustangs, means the same and will probably not throw any codes in the newer ones. If that area is corroded, then the rest of the pipe is probably messed up real nasty.
It's an anti-siphoning spring loaded thingy with a sharp edge that will capture non-metallic or fluted metal hoses in the pipe and will block garden hoses from getting in at all.
It will also stop you from removing a gas can with a plastic filler neck. Don't ask me how I know that.
You only need a flat blade screwdriver to push the flapper door out of the plastic hose attached to the gas can.
1:00 AM January 28th 1974, 23 deg F median of I-495 AKA Washington Beltway, 1973 Karmann-Ghia out of gas (hers). Round trip walk of 1/2 mile to the nearest exit and a flat blade screwdriver got me and my Fiance out of trouble. Very COLD walk, very WARM warm up later. A very good night. All's well that ends well. VERY well.
I told you not to ask, didn't I?
130 total pounds of Pitbull ruled by 8 pounds of cat.