What was found
AIRCRAFT WAS IN BASE MAINTENANCE: DURING EPSU SYSTEM TEST, (13WL) AFT L FAULT IS DETECTED. DISPLACED EMERGENCY OXYGEN CONTAINER FROM THE SERVICE CHANNEL OF THE AFT UTILITY AREA IAW A321 AMM 35-21-41-000-001-A REMOVED THE BATTERY OF THE EMERGENCY POWER SUPPLY UNIT (EPSU) (13WL), INSTALLED SERVICEABLE BATTERY OF THE EMERGENCY POWER SUPPLY UNIT (EPSU) (13WL). REINSTALLED EMERGENCY OXYGEN CONTAINER FROM THE SERVICE CHANNEL OF THE AFT UTILITY AREA.