As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU DETECTOR INOPERATIVE LIGHT ILLUMINATED ON THE GROUND, ALONG WITH MASTER CAUTION OVERHEAT DETECTOR. ACFT GROUNDED. TROUBLESHOT TO THE APU LOWER LOOP. REMOVED AND REPLACED THE APU LOWER FIRE DETECTION LOOP, OPS CHECKED NORMAL IAW AMM 26-10-01 AND 26-15-01.