As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MIA - CREW REPORTED AIRCRAFT UNABLE TO PRESSURIZE AFTER TAKEOFF. AFT OUTFLOW VALVE SHOWED FULLY CLOSED. EMERGENCY DECLARED FLIGHT RETURNED MIA LANDED WITHOUT INCIDENT. FOUND AFT OUTFLOW VALVE FULLY OPEN. AFT OUTFLOW WAS ON PLACARD. REPOSITIONED VALVE TO SEVEN PERCENT OPEN PER MEL 21-20A. SYSTEM GROUND CHECK NORMAL OPERATION.