As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN ALT LIGHT AND HORN CAME ON AT FL 220. FOUND GROUND AIR CONDITIONING CHECK VALVE OFF ITS HINGE ALLOWING PRESSURIZED AIR TO ESCAPE OVERBOARD. REPLACED FLAPPER DOOR/CHECK VALVE. DOOR SECURED AND OPERATES NORMALLY. REF AMM 21-21-01 PAGE 401. IPC REFERENCE # 21-21-01-01A.