As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFT CARGO SMOKE LOOP A WILL NOT TEST. LIKELY FAILED COMPONENT IS THE 50WU3, ELECTRONIC SMOKE DETECTION UNIT. TROUBLESHOT IAW TSM 26-19-01 STANDARDS. AMM: 26-19-34-04 PERFORMED BITE TEST ON MAIN CARGO SMOKE DETECTOR CONTROL UNITS, 50WU1, 50WU2 AND 50WU3. FAULT ON 50WU1 REAR "A" LIGHT NOT ILLUMINATED ON TEST. REMOVED AND REPLACED 50WU1 SMOKE DETECTOR CONTROL UNIT AND OPS CHECK GOOD.