As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED THAT THEY WERE UNABLE TO PRESSURIZE THE AIRCRAFT IN AUTO MODE OR IN MANUAL MODE. THE CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE EXTERIOR GROUND AIR CONNECTION VALVE FLAPPER MISSING. THE GROUND AIR CONNECTION VALVE WAS REPLACED. OPERATIONAL AND LEAK CHECKS WERE GOOD WITH NO FURTHER DEFECTS NOTED.