As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURN BACK . ACFT WOULD NOT PRESSURIZE AFTER TAKEOFF. NR 1 PACK DEFERRED ALREADY. NR 3 PACK VALVE SWITCH OPEN. CABIN ALTITUDE WARNING HORN SOUNDED CABIN AT 10,000 FT. A/C MODE SELECT SWITCH IN AUTO. BOTH OUTFLOW VALVES CLOSED. DUCT PRESSURE 42 PSI, CABIN CLIMBS WITH AIRCRAFT, DIFF PRESS "0". WHEN NR 3 PACK SELECTED THERE WAS NO CHANGE IN DUCT PRESSURE OR CABIN VERTICAL SPEED. FOUND NR 3 PACK FLOW CONTROL AND SHUTOFF VALVE WOULD NOT OPEN WHEN COMMANDED. REPLACED VALVE PER MM 21-51-1 OPS CHECKED NORMAL.