As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU AUTO SHUTDOWN WHEN PACKS TURNED OFF, PRIOR TO ENG START. PNEU PRESS DROPS CLOSE TO ZERO WHEN PACKS SWITCHED OFF. REPEAT W-U. BIT CONTROLLER. FOUND REVERSE FLOW FAULT. REPLACED APU. CLEARED CONTROLLER FAULT. WITH ELEC AND PNEU LOAD, UNABLE TO DUPLICATE SHUTDOWN, TURNING PACKS ON-OFF AND MOTORING ENGS.