As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO PHL DUE TO THE FLIGHT CREW REPORTING THAT THE AIRCRAFT WOULD NOT PRESSURIZE. MAINTENANCE HAD PREVIOUSLY DEACTIVATED THE SKIN AIR OUTLET VALVE DUE TO A VENT AVIONICS FAULT MESSAGE PER MEL 21-07. MAINTENANCE RE-ACTIVATED THE SYSTEM AND RAN THE AVEC TEST PER AMM 21-26-00-710-001A WITH NO FAULTS NOTED. MAINTENANCE RAN PRESSURIZATION TEST PER AMM 05-53-00 AND AIRCRAFT PRESSURIZED SUCCESSFULLY.