What was found
APU INOPERATIVE LIGHT ILLUMINATED FOR A SHORT TIME AFTER TAKEOFF, FOLLOWED QRH. ACFT GROUNDED. PERFORMED FIM TASK 26-10, TASK 801 NO FAULTS FOUND. PERFORMED FIRE SYS & LIGHT CHECK NO DEFECTS NOTED. RUN APU FOR FIVE MINUTES, SHUTDOWN. PERFORMED FIM TASK 26-10 TASK 801 STILL NO FAULTS. PERFORMED FIM 26-10, TASK 801. NO FAULTS FOUND. REMOVED AND INSTALLED NEW APU TAIL PIPE DETECTOR IAW 26-15-01-000-802. PERFORMED APU DETECTOR SYS TST SYS CHECKED NORMAL.