As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED THE AIRCRAFT WOULD NOT PRESSURIZE AFTER TAKEOFF. CREW ELECTED TO RETURN TO DAY AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND FOUND THE OVERBOARD EXHAUST VALVE IN THE OPEN POSITION. MAINTENANCE CLOSED THE OVERBOARD EXHAUST SHUT-OFF VALVE IN THE CLOSED POSITION AS PER THE MEL 21-24-06-2. AIRCRAFT PRESSURE CHECK WAS GOOD. AIRCRAFT WAS RELEASED FOR SERVICE.