As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PRE FLIGHT CREW REPORTED THAT THE APU AUTO SHUTDOWN ON THE GROUND FOLLOWED BY AN ELECTRICAL BURNING SMELL IN THE COCKPIT AND CABIN. RAMP WORKERS REPORTED A LARGE FIREBALL AND SMOKE COMING FROM THE APU EXHAUST CONE WHEN APU SHUTDOWN. MAINTENANCE REMOVED AND REPLACED THE APU.