What was found
PHX-LAX - FLT 15 - ON CLIMB-OUT AIRCRAFT LOST RT PACK, CREW WAS UNABLE TO PRESSURIZE CABIN. LEFT PACK WAS ON MEL. AIRCRAFT RETURNED TO PHX WITHOUT FURTHER PROBLEMS. MAINTENANCE REPLACED FWD OUTFLOW VALVE AND E/E COOLING EXHAUST VALVE PER MM 21-58-31, PAGE 401. PRESSURIZED AND TESTED PER MM 5-51-91, CHECK 5. AIRCRAFT RETURNED TO SERVICE.