What was found
APU WON'T START. TRIED 3 TIMES GOT APU DOOR MSG. DOOR BREAKER IN, DOOR OPENS BUT APU WILL NOT START, DOOR IS CLOSED AND APU INOPERATIVE. WHEN APU MASTER SWITCH TURNED TO START RUN LIGHT ONLY FLASHES ONCE, SHOULD FLASH TWICE. THIRTY SECONDS LATER, APU DOOR STATUS MSG. APU DOOR OBSERVED TO BE PHYSICALLY OPEN. DOOR CLOSES WHEN SWITCH SELECTED OFF. DURING TROUBLESHOOTING FOUND APU DOOR OPEN SWITCH BAD. REPLACED APU INLET DOOR OPEN SWITCH S506. APU OPERATION NORMAL.