As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PASSING THROUGH 10K FEET NOTICED LACK OF AIRFLOW. SWITCHED TO APU AIR. NO HELP PASSING THROUGH 22K FEET. RECEIVED 'CABIN HI ALT' MESSAGE. PERFORMED EMERGENCY DESCENT. DECLARED EMERGENCY AND RETURNED TO ORD. REMOVED AND REPLACED DUCT. REMOVED AND REPLACED CHECK VALVE (P/N 92E20-3) AND WATER SEPARATOR, P/N 601R95251-15. (M)