What was found
UNABLE TO CONTROL CABIN ALTITUDE. CABIN AT 9400 FT, AIRCRAFT AT 11000 FT. THE SAME IN AUTO 1 AND AUTO 2. IN MANUAL, CABIN ALTITUDE INCREASED BY 200 FPM. RETURNED TO FIELD. THE LEFT AND RIGHT OUTFLOW VALVES ACTUATORS AND THE CABIN PRESSURIZATION ALTIMETER WERE REPLACED. THE AIRCRAFT WAS RETURNED TO SERVICE. THE AIRCRAFT RETURNED TO THE FIELD AGAIN FOR A SIMILAR PROBLEM ON 30 OCT, 2002. THE GROUND EQUIPMENT COOLING VALVE WAS FOUND TO BE THE ROOT CAUSE OF THE PROBLEM.