As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED THAT DURING FLIGHT, THEY WERE UNABLE TO PRESSURIZE THE AIRCRAFT WITH EITHER AUTO-PRESSURIZATION OR MANUAL PRESSURIZATION MODES. THE AIRCRAFT RETURNED TO FIELD. MAINTENANCE FOUND THE EXTERIOR GROUND AIR CONNECTION VALVE FLAPPER MISSING. MAINTENANCE DEFERRED THE ITEM. THE DEFERRAL WAS CLEARED ON 5/2/2012 BY REPLACING THE GROUND AIR CONNECTION VALVE. OPERATIONAL AND LEAK CHECKS WERE GOOD WITH NO FURTHER DEFECTS NOTED.