What was found
(CAN) CABIN PRESSURIZATION FAILURE. NO DIFF BUILD UP AND CABIN ALTITUDE STARTED CLIMBING AT THE SAME RATE AS THE AIRCRAFT ROC. NO CAUTION OR WARNING LIGHT WHILE PASSING 6000 FT. THE PRESSURIZATION FAILURE WAS NOTICED. LEVELED OFF AT FL 70, QRH WAS CARRIED OUT. PRESSURIZATION WAS NOT REGAINED. AIR TURN BACK CARRIED OUT. MAINTENANCE PERFORMED A FUNCTIONAL TEST AND OBSERVED CPC/OFV DATA FAIL MESSAGE ON CDS. SUSPECTED THAT THE CPC TO BE FAULTY. SAME REMOVED AND REPLACED. SYSTEM CHECK CARRIED OUT SATISFACTORY. AIRCRAFT RETURNED TO SERVICE. (TC# 20190301001)