As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ETOPS SIGNIFICANT SYSTEM: AFT CARGO FIRE DETECTION "B" LOOP WILL NOT TEST. ACCOMPLISHED BITE TEST ON AFT CARGO ELECTRONICS UNIT. FOUND B1 DETECTOR AT FAULT. REMOVED AND REPLACED AFT CARGO B1 SMOKE DETECTOR IAW B737-800 MM 26-16-01. OPS CHECK NORMAL.