What was found
APU INOP, 3 ATTEMPTS. PERFORMED APU ECU BIT CHECK. MINI FLAGS 21,26,29,36,38,42,48 AND 66 DISPLAYED. FURTHER T/S REQUIRED. SUCCESSFUL APU START 1 OF 3 ATTEMPTS. IAW AMM 49-11-00 + 49-61-05. REMOVED & REPLACED APU ECU. OPS CK GOOD. 3 SUCCESSFUL STARTS. DURING 3RD RUN APU AUTO SHUTDOWN. ECU FAULTED REVERSE FLOW REF AMM 29-22-02 & 49-61-05. REMOVED AND REPLACED REVERSE FLOW MODULE IAW AMM 49-53-08, NO FIX REMOVED AND REPLACED APU INLET TEMP SENSOR IAW AMM 49-61-03, NO FIX. SUSPECT FAULTY CHECK VALVE. PERFORMED FIM 49-11 FAULT TREE A04 AND FOUND ECU TO BE FAULTY, NO ECU IN STOCK AT CVG, REMOVED AND REPLACED APU& ECU IAW AMM 49-61-01. OPS CHECK GOOD AT THIS TIME.