As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PREFLIGHT CHECK, FOUND AFT CARGO FIRE DETECTOR CIRCUIT BREAKER POPPED AND WOULD OT RESET. TROUBLESHOT SYSTEM. FOUND AFT CARGO SMOKE DETECTOR ELECTRONIC UNIT AT FAULT. REMOVED AND REPLACED ELECTRONIC UNIT. SYSTEM CHECKED NORMAL IAW 26-16-02. AIRCRAFT RETURNED TO SERVICE. (SDR040012)