As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
"APU DET INOPERATIVE" LIGHT ILLUMINATED STEADY ON APU FAULT/INOPERATIVE TEST FAIL. ACFT GROUNDED. CONFIRMED FAULT, APU WILL NOT TEST FIRE DETECTION TEST. FOUND APU TAIL CONE FIRE DETECTOR DEFECTIVE. REPLACED APU TAIL CONE FIRE DETECTOR REF AMM 26-15-00, GROUND FUNCTIONAL CHECK NORMAL, REF AMM 26-15-00.