As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT GATE, APU OPERATING ON BUSS, NO BLEED, APU FIRE DETECTOR INOPERATIVE LIGHT ILLUMINATED. ACFT GROUNDED. VERIFIED DISCREPANCY. REMOVED AND INSTALLED TAIL PIPE DETECTOR REF MM 26-15-01-400-802. PERFORMED APU FIRE DETECTOR, OPS CK REF 26-15-00-710-801.