As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB-OUT, LEFT OUTFLOW VALVE STOPPED CLOSING IN AUTO. CABIN STARTED CLIMBING AT MAX RATE WENT TO MANUAL IT STILL DID NOT CLOSE CABIN REACHED 16,000 FT. ALL CREW ON OXYGEN. CONTINUED ATTEMPTING TO CLOSE OUTFLOW VALVE MANUALLY UNTIL IT BEGAN CLOSING TO GET CONTROL OF CABIN. REMOVED AND REPLACED LEFT OUTFLOW VALVE ACTUATOR PER B747 MM 21-31-08, OPS CHECKED NORMAL PER B747 MM 21-31-00.