As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) FOLLOWING DEPARTURE, THE ACFT FAILED TO PRESSURIZE IN AUTO OR MANUAL MODE. THE ACFT RETURNED TO POINT OF DEPARTURE AND LANDED WITHOUT FURTHER PROBLEM. THE GROUND AIR CONNECTION CHECK VALVE WAS FOUND TO BE STUCK IN THE OPEN POSITION. MAINT CLEANED AND LUBRICATED THE VALVE TO RESTORE PROPER FUNCTION. THE SYS WAS GROUND CHECKED SERVICEABLE AND THE ACFT RETURNED TO SERVICE WITH NO FURTHER PROBLEM REPORTED.