As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FIRE INDICATION IN FLIGHT @ AROUND 13K APU SOV PUSHED IN APU EXT BOTTLE PUSHED, CREW RESPONDED QUICKLY AND CORRECTLY VIA MEMORY ITEMS AND QRH UTILIZING THE FIRE EXTINGUISHING BOTTLE. AIRCRAFT RETURNED TO FIELD, GVI WAS PERFORMED PER AMM 05 INSPECTIONS, NO INDICATIONS OF ANY FIRE WERE FOUND. FURTHER TROUBLESHOOTING OF SYSTEM WITH FIM 26-12-00 FOUND THAT THE APU FIRE DETECTOR FAILED RESISTANCE CHECK CAUSING THE FALSE WARNING AND INDICATION. APU FIRE BOTTLE AND CARTRIDGE REPLACED.