What was found
INSPECTION TYPE: NA ATB-AC OUT OF SERVICE. EMERGENCY DECLARED, LANDED W/OUT INCIDENT, NO EMERGENCY EQUIPMENT USED. APU FAULT, APU SHUTDOWN WHICH RESULTED IN AN AIR TURNBACK. DOWN TO ONE GEN, AS FAULT LIGHT NEVER WENT OUT. APU ECU BITE CHECK SHOWED "DC POWER LOSS" STARTED APU TWICE-NO HELP. REPLACED APU ECU, PER MM 49-61-05. RAN APU, OPERATIONS CHECK GOOD. IPC 49-61-05-02C.