As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: WHILE ACCOMPLISHING EMERGENCY POWER SUPPLY UNIT SYSTEM TEST A FAULT MESSAGE FOR THE MID LEFT EMERGENCY POWER SUPPLY UNIT (15WL) DISPLAYED. MAINTENANCE REPLACED EMERGENCY POWER SUPPLY UNIT 15WL (AALA202307101411) AND THE BATTERY (AALA202307102334) PER AMM 33-51-38. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED.