What was found
DESCENDING THROUGH 20000 FT, OVERHEAT DETECTION LIGHT MASTER CAUTION LIGHT ILLUMINATED FOLLOWED BY APU DETECTION INOPERATIVE LIGHT. FOLLOWED CHECK LIST. DID NOT START APU. FAULT LIGHT EXTINGUISHED ITSELF. ACFT GROUNDED. TESTED APU FIRE DETECTION SYS IAW AMM 26-15-00/501. NO FAULTS FOUND. COULD NOT DUPLICATE DEFECT. RAN APU 3 STARTS/STOPS WITH NO FAULTS FOUND. NO MAINTENANCE HISTORY WITHIN THE PAST 30 DAYS. TSR ISSUED. ACCOMPLISHED APU DETECTOR RESISTANCE CHECK AND INSPECTION AS REQUESTED IN TSR. NO DEFECTS FOUND.