What was found
APU FAULTED, RAN QRH CHECKLIST. APU FAULT LIGHT REMAINED ON. SINCE SINGLE GEN OPS RETURNED TO ICT. CREW REPORTED THAT APU DID AUTO SHUTDOWN AND IAW CHECKLIST THEY WERE NOT DIRECTED TO TAKE SWITCH TO OFF. THEY TRIED TO RESTART WITH NO LUCK. WHEN ACFT ARRIVED AT GATE APU DOOR WAS OPEN WITH APU OFF. PERFORMED BITE TEST OF APU ECU AND GOT FAULT ON LAST FLT OF "DOOR SYS" SUSPECT THIS WAS INDUCED BY TRYING TO RESTART. NEXT FAULT WAS "REVERSE FLOW". STARTING T/S. REMOVED AND REPLACED APU SURGE VALVE REMOVED AND REPLACED WHILE PERFORMING BITE TEST ON APU ECU GOT MINI FLAG 14, FAILED TOTAL PRESSURE SENSOR. REMOVED AND REPLACED SAME IAW MM 49- 53-03-2. RAN APU SEVERAL MINUTES, WITH SEVERAL DIFFERENT PNEUMATIC AND ELECTRICAL LOADS. APU RUNS FINE. AFTER SHUT DOWN, CHECKED APU ECU AND FOUND NO FAULTS. RAN APU AGAIN TODAY AND FOUND NO FAULTS. APU OPS GOOD IAW JRH. IDG WAS REPLACED WITH 24-20 2181. REMOVED AND REPLACED LT ENG IDG IAW MM 24-11-01-4. LEAKS AND OPS GOOD.