What was found
PHX/OAK - FLT 0450 - AIR TURNBACK. PRESSURIZATION WENT TO MAXIMUM DIFFERENTIAL ON CLIMB-OUT. UNCONTROLLABLE IN AUTO MODE. CREW REPORTED NO ENGINE BLEED PROBLEMS. INSPECTED CARGO DOOR SEALS AND PERFORMED OPERATIONS CHECK OF CABIN PRESSURE CONTROLLER AND OUTFLOW VALVE. NO DISCREPANCIES NOTED. REMOVED AND REPLACED CPC PER MM 21-31-21 AS PRECAUTION. OPERATIONS CHECK AND LEAK CHECK OF FIRST AUXILIARY STATIC GOOD. PART TOTAL CYCLES, 31,351. (X)