As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FIRE WARNING INDICATION AFTER APPROXIMATELY 15 MINUTES OF APU OPERATION ON GROUND. CREW DISCHARGED FIRE EXTINGUISHING SYSTEM AND SHUT DOWN APU. TROUBLESHOT PROBLEM TO A BAD BOTTOM APU HEAT DETECTOR. REPLACE HEAT DETECTOR WITH NEW UNIT. REPLACE FIRE BOTTLE/SQUIB ASSEMBLY WITH OVERHAULED UNIT. NO ACTUAL OVERHEAT/FIRE OCCURRED.