As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU DETECTOR INOPERATIVE, ILLUMINATED IN CRUISE AT 3500' WITH THE APU NOT RUNNING SAT -48 DEG. CEL. ACFT GROUNDED. IAW WDM 26-11-31, AMM 26-15-01-000-802. ACCOMPLISHED ALT DEACTIVATION REF CH 5 TABLE 3 ITEM 1. ACCOMPLISHED RESISTANCE CHECKS OF APU FIRE DETECTORS. DETECTORS M1755, M1756 ARE WITHIN LIMITS. DETECTOR M1925 IS OUT OF LIMITS. REMOVED AND REPLACED M1925 DETECTOR.