As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE AT THE GATE WITH THE APU POWERING THE BUSSES, THE APU POWER DISCONNECTED FROM THE AIRCRAFT. THE APU GEN OFF BUS ILLUMINATED AND THE APU POWER WOULD NOT RECONNECT. THE APU GEN SHOWS NO VOLTS OR FREQS. ACFT GROUNDED. PERFORMED APU OPERATIONAL TEST IAW AMM 49-11-00-710-801. OPS CHECKED GOOD. VOLTS AND FREQ REMAINS NORMAL.