As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA - CLIMBING THROUGH 20,000 FT REAR OUTFLOW VALVE DUMPS. DESCENDING BELOW 20,000 FT PRESSURIZATION WORKS NORMALLY. RETURNED TO FIELD. LANDING UNEVENTFUL. FOUND "CPC/OF VALVE DATA FAIL" IN CDS. REMOVED AND REPLACED AFT OUTFLOW VALVE IAW Q400 AMM TASK 21-31-16-000-801 AND 21-31-16-400-801. FUNCTIONAL TEST OF THE PRESSURIZATION CONTROL IS GOOD. FAULT CODE CLEARED AND DID NOT REAPPEAR. RETURNED TO SERVICE.