As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING INSP OF THE REAR PRESSURE BULKHEAD, THE FLAPPER VALVES ON THE MAIN CABIN NON RETURN VALVE WERE FOUND TO BE STUCK IN THE OPEN POSITION. FURTHER INSP REVEALED THAT THE CTR SHAFT HAD TOOLING MARKS ON IT AND THE SHAFT WAS CRACKED. THE SPRINGS THAT FORCE THE FLAPPER VALVES CLOSED WERE ALSO BROKEN.