As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING OPERATIONAL TEST OF EMERGENCY LIGHTS, RECEIVED MAINTENANCE MESSAGE FOR FLOOR PROXIMITY EMERGENCY MARKING SYSTEM RIGHT CROSSAISLE LIGHT EPSU (12WL). AIRCRAFT WAS GROUNDED. REMOVED AND REPLACED EMERGENCY POWER SUPPLY UNIT BATTERY AT 12WL IAW AMM 33-51-38. OPERATIONAL CHECK GOOD.