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, DURING CLIMBOUT CABIN VERTICAL SPEED INSTABLE -450 TO +900, OUTFLOW VALVE FLUCTUATING ON SYSTEM 2. THE AIRCRAFT LANDED SAFELY AT PHL WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE #2 CABIN PRESSURE CONTROLLER 12HL AND ACCOMPKISHED A SATISFACTORY OPERATIONAL CHECK PER TSM 21-31-00-810-868-A WITH NO FAULTS NOTED.