What was found
AIRCRAFT WAS GROUNDED: AIRCRAFT PRESSURIZATION ISSUE. CABIN ALTITUDE WAS STUCK AT 0 DURING CLIMB TO 10000 FT. HAD NO CABIN RATE OF CLIMB DURING THE CLIMB. OUTFLOW VALVE POSITION SHOWED SLIGHTLY OPEN BUT NEVER MOVED UNTIL MANUAL OPERATING IT TO FULL OPEN PRIOR TO LANDING PER QRH. DECESION MADE TO RETURN TO JFK. EMERGENCY DECLARED AND AIRCRAFT LANDED OVERWEIGHT WITHOUT INCIDENT. MAINTENANCE REPLACED PACK FLOW TEMP SENSOR PER AMM 21-51-43.