What was found
PILOT REPORTED, A/C DID AIR TURNBACK DUE TO APU AUTO-SHUTDOWN IN FLIGHT. THIS LEFT A/C WITH ONLY ONE OPERATING GENERATOR DUE TO LEFT ENGINE GENERATOR BEING ON DEFERRAL (M-97875). BITE CHECKED APU, FOUND REVERSE FLOW SHUTDOWN FAULT WITH NO LRU FAULTS. REPLACED LEFT ENG IDG IAW MM 24-11-1, OPS CKS OK AND THIS CLEARED M 97875. REPLACED APU SURGE BLEED VALVE IAW MM 49-53-1 AND APU FLOW SENSOR MODULE IAW MM 49-53-8, OPS CHECKS OK.