As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING C4 C-CHECK, INSPECTION NOTED THAT THE EMERGENCY LIGHT TEST THROUGH THE CENTRALIZED FAULT DISPLAY UNIT FAILED OPERATIONAL CHECK. MAINTENANCE REMOVED AND REPLACED THE EMERGENCY POWER SUPPLY UNIT BATTERY IN ACCORDANCE WITH AMM 33-51-38-000-001.