As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PASSENGER BOARDING AT KDTW, GATE B6, THE APU BEGAN TO MAKE A HIGH PITCH WHINE, FOLLOWED BY AN AUTO SHUTDOWN. SMOKE WAS REPORTED COMING FROM THE REAR OF THE AIRCRAFT. THE FLIGHT CREW HALTED BOARDING AND DEPLANED THE PASSENGERS. MAINTENANCE DETERMINED THE APU HAD FAILED. THE AIRCRAFT WAS INSPECTED AND THE APU DEFERRED PER MEL. THE AIRCRAFT WAS RELEASED FOR SERVICE. THE APU WAS SUBSEQUENTLY REPLACED AT THE NEXT HANGAR VISIT.