As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DURING EMERGENCY LIGHTING SYSTEM TEST, FAULT MESSAGE FOR EPSU INT LOAD FWD L DISPLAYED. MAINTENANCE REPLACED EMERGENCY POWER SUPPLY UNIT 10WL BATTERY AND ACCOMPLISHED A SATISFACTORY OPERTATIONAL CHECK ODFTHE EMERGENCY LIGHTING SYSTEM WITH NO FAULTS NOTED.