As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED CABIN ALTITUDE WARNING AT 19,000 FT. CREW FOLLOWED PROCEDURES AND MASKS DEPLOYED IN CABIN. EMERGENCY DECLARED, FLIGHT RETURNED CCS LANDED WITHOUT INCIDENT. ACFT REMOVED FROM SERVICE. MX FOUND AIR CHECK VALVE STUCK IN THE OPEN POSITON. RESET VALVE AND ACCOMPLISHED PRESSURIZATION CHECK. REPLACED O2 CONTAINERS. SYSTEM GROUND CHECK NORMAL OPERATION.