What was found
ON CLIMB-OUT, LEFT AIR MIXING VALVE STUCK TOWARD HOT RESULTING IN DUCT TRIP AND PACK TRIP, CABIN ALTITUDE WOULD NOT HOLD AT 25,000 CABIN ALTITUDE AT 11,000, ON RETURN TO MSP WAS 9,000. REMOVED AND REPLACED LEFT MIXING VALVE, OPS CHECK GOOD IN MANUAL MODE IAW MM 21-61. (M)