What was found
AT 22000 FT CABIN DIFF AT 5.9 PSI. CABIN ALT 3800 FT. DIVERTED. LANDING UNEVENTFUL. REMOVED FROM SERVICE. INTERROGATED CDS AND FOUND CP CONTROLLER, INDICATION MOD AND CPC/OF VALVE DATA FAIL FAULTS. R/R AFT OUTFLOW VALVE IAW Q400 AMM 21-31-16-000/400-801. NO HELP. REMOVED & REPLACED CABIN INDICATION MODULE IAW Q400 AMM 21-31-11-000/400-801. ALL FAULTS CODES CLEARED & DID NOT RETURN. FUNCTIONAL TEST OF PRESSURIZATION CONTROL IS GOOD IAW Q400 AMM 21-31-00-720-801. PRESSURIZATION OF A/C IN MANUAL MODE IS GOOD IAW Q400 AMM 21-00-00-863-801/802. RETURNED TO SERVICE.