APU Inlet Door Position

APU was not starting. History stated that it would make approximately 30% and shutdown. I didn't try a start in the failed condition.

The ECU was showing a inlet door failure.
  
     
  
The ECU needs to receive a ground signal indicating the inlet door is open. I'm surprised a start would be initiated with the door not showing fully open.

I "fooled" the system and had a successful start.

Looking at the schematic.....
  
     
  
If you pull relay K176 and jumper B2 (28 VDC) to B1 (door actuator "open" coil), you can command the door to open. (B2 to B3 will close it.)
  
     
  
With the door open, relay K547 should be energized which provides the ground signal to the ECU on pin 2. I visually verified the door was open and jumped pin 2 to ground.
  
     
  
With the good start, the only conclusion would be a bad or malfunctioning door sensor. This switch is made when a magnetic target is in close proximity.

The switch can be verified as good or bad by checking for a ground at pin 7 of K547 with the door open.

I didn't see the issue first hand, but there was a busted rivet on the target. Along with a target gap that was over limits.
     
      
 

Good troubleshooting! Sometimes you look at something, not obviously broken, but broken just enough to cause problems. And then sometimes you find something big, and wonder how it hasn't caused a problem!