As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT THE GATE WHILE APU WAS RUNNING, THE PILOT NOTICED THE SMELL OF BURNT ELECTRICAL WIRING FOLLOWED BY APU FIRE WARNING LIGHT. THE PILOT PULLED THE FIRE HANDLE AND DISCHARGED THE FIRE BOTTLE. MAINTENANCE CLEANED AND INSPECTED THE WHEEL WELL AREA FINDING BURNT WIRING IN THE APU SHROUD. APU WAS LISTED ON MEL.