For older Mustangs with EVAP leaks also check:
- For a rusted out filler neck. Especially under the plastic rock shield (if present).
- Dry rotted gas tank filler neck grommet.
Gas vapors are very hard on rubber parts. +1 on checking for dry rotted vacuum lines in the engine bay.
IMO the stuck open flapper valve is not causing the DTC code.
More information about the gas tank filler neck grommet. https://www.allfordmustangs.com/forum...tank-help.html