As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) COULD NOT PRESSURIZE THE AIRCRAFT. AIRCRAFT LANDED BACK, FOLLOWING INVESTIGATION, MAINTENANCE FOUND IN FAULT THE LT PILOT PRESSURE REGULATING VALVE P/N 392454-3-1 S/N 6549 AND LT PRESSURE REGULATOR VALVE P/N 392714-1-1 S/N 4522. BOTH VALVES REPLACED AND AIRCRAFT DISPATCHED SERVICEABLE.