As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT RETURNED TO FIELD WITH CABIN PRESSURE FAULTS. THE EQUIPMENT COOLING OVERBOARD EXHAUST VALVE FAILED. THE VALVE WAS NOT DEACTIVATED PER THE MEL CAUSING A LOSS OF CABIN PRESSURIZATION ON THE SUBSEQUENT FLIGHT. THE VALVE WAS RE-DEFERRED PER THE MEL AND REPLACED AT THE NEXT MAINTENANCE OPPORTUNITY WITH NO FURTHER PROBLEMS. LINE QA IS CONDUCTING A INVESTIGATION INTO THE CIRCUMSTANCES.