What was found
DURING CLIMBOUT, THE LEFT MIX VALVE IN BOTH MANUAL AND AUTO, PACK TEMPERATURE STUCK AT 40 DEGREES. CABIN COULD NOT HOLD PRESSURE BELOW 10,000 FT WITH ONLY RIGHT PACK OPERATING IN AUTO OR STANDBY. THE CREW ELECTED TO RETURN TO POINT OF ORIGIN. REMOVED AND REPLACED THE LEFT PACK VALVE, PACK OPERATIONAL CHECKS NORMAL, LEAK CHECKS GOOD.