What was found
LEAVING FL210, WE RECEIVED APU CAUTION EICAS COMPLETED CHECKLIST & HAD DASHES ON OIL TEMP. FOLLOWED BY APU FAULT FOLLOWED BY APU SHUTDOWN. COMPLETED CHECKLIST FOLLOWING APU SHUTDOWN. UPON INSPECTION FOUND APU AIR INLET DOOR MISSING. REMOVED APU INLET DOOR ACCESS PANEL IAW AMP J49-12-03 FOR HINGE REMOVAL. STARTED APU INLET DOOR HINGE REMOVAL IAW AMP J49- 12. 3, BOLTS STILL REMAIN TO BE REMOVED (411051DK) COMPLETED REMOVAL OF APU AIR INLET DOOR IAW AMP J49-12-03 (DS136211) INSTALLED DOOR ACTUATOR & POSITION SENSOR IAW 49-12 UP TO APU INLET DOOR RIGGING. (JS556289) FINAL INSTALL OF APU INLET DOOR IS DOCUMENTED ON WP P477811020260706, NR 002 WITH REF TO ERA 635573-14 REV A. (LP# 8908594; AIRCRAFT WAS GROUNDED)