As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DURING PERIODIC SERVICE CHECK MID CABIN EXIT SIGN OUT / NOT ILLUMINATING. MAINTENANCE FOUND EMERGENCY POWER SUPPLY UNIT 15WL NOT TESTING AND REPLACED IT PER AMM 33-51-38. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED.