As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WITH APU ON AND PROVIDING POWER EXTERNAL POWER DISCONNECTED CAUSED ALL POWER TO BE LOST. BATTERY PROVIDING POWER. APU GEN OFF BUSS LIGHTS ON, RECONNECTED APU TO BUSSES AND CDS FAULT LIGHT ILLUMINATED FOR APPROX 3 TO 4 MIN. THEN WENT AWAY. ACFT GROUNDED. PERFORMAED APU OPS TEST, START, REF AMM 49-11-00-800-801. BLUE LIGHT EXTINGUISHED. NO FAULTS NOTED. CDS LIGHT NORMAL DURING DEU BOOT PROCESS REF FIM TASK 31-62, TASK 801, PARA B.