As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LAX FLIGHT NR 2334: CABIN ALT CLIMBING EXCESSIVELY. NOTICED FAULT LIGHT ILLUMINATED ON PRESSURE CONTROLLER. RAN EMERGENCY CHECKLIST. HAD THEM RUNNING IN MANUAL MODE. MAINTENANCE REMOVED AND REPLACED AFT OUTFLOW VALVE IAW Q400 AMM TASK 21-31-10-000-801 AND 21-31-10-400-801. FUNCTIONAL TEST GOOD IAW 21-31-00-720-801. AIRCRAFT RETURNED TO SERVICE.