What was found
AIRCRAFT WAS GROUNDED. EMERGENCY DECLARED. LANDING UNEVENTFUL. DURING DESCENT THE NR 1 PRESSURIZATION CONTROLLER FAILED WITH THE NR 2 PREVIOUSLY PLACARDED. DIFFERENTIAL PRESSURE WENT TO MAXIMUM AND CABIN ALTITUDE WENT BELOW SEA LEVEL. CREW REVERTED TO MANUAL PRESSURIZATION CONTROL. MAINTENANCE FOUND THE OUTFLOW VALVE NOZZLE STICKING IN THE CLOSED POSITION. CLEANED DIRT AND DEBRIS FROM NOZZLE AND ACCOMPLISHED OPERATIONAL CHECK IAW AMM 21-32-05. SYSTEM CHECKS NORMAL.