As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR INTERRUPT FOR INABILITY TO PRESSURIZE AIRCRAFT DUE TO AFT OUTFLOW VALVE INOPERATIVE. AN. EMERGENCY WAS DECLARED. THE FLIGHT RETURNED TO DEPARTURE WITHOUT FURTHER INCIDENT. AIRCRAFT WAS GROUNDED. REPLACED THE AFT OUTFLOW VALVE IAW AMM 21-31-03 AND PERFORMED SATISFACTORY OPERATIONAL CHECK.