What was found
(CAN) DURING INVESTIGATION INTO PILOT REPORTED DEFECT, FULL COLD SELECTED, LT SIDE HOT, MAINTENANCE PERFORMED A FUNCTIONAL CHECK OF THE MIXING VALVE AND FOUND THE BUTTERFLY VALVE TO BE STAYING IN THE OPEN POSITION WHEN SELECTED CLOSED. VALVE ASSY REMOVED. THE BUTTERFLY VALVE ON THIS ASSY HAS BEEN IN CONTACT WITH THE INNER VALVE BODY, SCORING BOTH. APPEARS THAT THE SPLINED DRIVESHAFT SHIFTED UP TOWARDS THE MOTOR AND PULLED THE BUTTERFLY VALVE WITH IT, CAUSING THE SCORE MARKS AND EVENTUALLY STRIPPING THE BUTTERFLY/SHAFT.