As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL - FL 445 - CREW COULD NOT GET CABIN TO PRESSURIZE. FLIGHT RETURNED TO PHL AND LANDED WITH NO FURTHER INCIDENT. MAINTENANCE FOUND THE FLEX DUCT DISCONNECTED FROM THE LEFT ACM. THE DUCT WAS RESECURED, AND THE AIRCRAFT GROUND PRESSURIZED USING LEFT, RIGHT AND THEN BOTH ACM'S. (M)