What was found
EN ROUTE EXPERIENCED CABIN PRESSURE LOSS, DONNED O2 MASKS & EXECUTED EMERGENCY DESCENT. MANUALLY DEPLOYED CABIN O2 MASKS (MASKS WERE NOT USED), STANDBY & MANUAL DID NOT WORK, TRIED AGAIN LATER AND STANDBY DID WORK. LANDED AT PHX WITHOUT FURTHER INCIDENT, EMERGENCY WAS NOT DECLARED. MX RE-STOWED MASKS AND REPLACED AFT OUTFLOW VALVE IAW BMM 21-31-11-4. PERFORMED HEATER TEST OF AFT OUTFLOW VALVE AND CABIN PRESSURE TEST, BOTH TESTS WERE GOOD. FUNCTIONAL TEST OF OUTFLOW VALVE SN 860743 CONFIRMED REASON FOR REMOVAL FOR FAILING PRETEST FINDING MOTOR AND GEARBOX ASSEMBLY INOPERATIVE AND SLUGGISH. REPLACED SERVOMOTOR AND BRAKE ASSEMBLY AND 2 EACH ANNULAR BALL BEARINGS. RETESTED UNIT IAW 21-33-00 REV. 19.