What was found
CLIMBING THROUGH 20,000 FEET, CABIN ALTITUDE WARNING HORN SOUNDED, CABIN ALTITUDE WAS AT 12,000 FEET. COM PROCEDURES FOLLOWED WITH DESCENT TO 10,000 FEET. FUEL WAS DUMPED AND AIRCRAFT LANDED UNEVENTFULLY. AT TIME OF EVENT, 3 PACS WERE OPERATING, BLEEDS OPEN, ISOLATION VALVE OPEN, OUTFLOW VALVES SHOWED CLOSED AND AUTO CONTROLLER WAS IN AUTO. ALTITUDE CONTROLLER WAS SET AT 34,000 FEET, BARO SET AT 29.92. ON TAXI IN OUTFLOW VALVES OPENED MANUALLY. THERE WERE NO DOOR OPEN LIGHTS ILLUMINATED. NO GROUND SAFE RELAY LIGHT ON DURING DESCENT. AT 10,000 FEET CABIN ALTITUDE WAS 7,000. REPLACED RIGHT OUTFLOW VALVE.