What was found
(CAN) DURING CRUISE AT FL200,. ALL PRESSURIZATION INDICATIONS WERE NORMAL UNTIL CREW NOTICED PRESSURE CHANGE IN CABIN. CABIN RATE OF CLIMB SHOWED +3000FT/MIN, CREW DECIDED TO USE OXYGEN MASKS, SHORTLY AFTER "CABIN PRESS" WARNING LIGHT ILLUMINATED. CREW CONDUCTED APPROPRIATE RECALL, MADE A PAN-PAN CALL AND STARTED AN EMERGENCY DESCENT TO 10,000 FT. THEN RAN THROUGH QRH CHECKLIST AND CONTACTED CABIN CREW TO CHECK IF EVERYBODY WAS OK IN CABIN. FLIGHT CREW CONTACTED COMPANY, ADVISED THEM OF SITUATION AND RETURNED TO BASE AT 10,000 FT WITH NO FURTHER INCIDENT. "CDS FAULT", "CPC/OF VALVE DATA FAIL" INITIALLY FOUND ATTEMPTED TO CLEAR BUT INDUCED FAULT "CP CONTROLLER FAIL" UNABLE TO CLEAR FAULTS. TROUBLESHOOTING C/O, CABIN PX CONTROLLER & AFT OUTFLOW VALVE REPLACED.