What was found
ENROUTE TO 066 @ 350FL- MASTER CAUTION W/ APU INOP DET ILLUMINATED. GROUNDED - YES. DIVERSION TIME 107 MINUTES - CONFIRMED APU INOP DET LIGHT ILLUMINATED. DOES NOT EXTINGUISH W/MASTER CAUTION. VERIFIED DISCREP. HARD FAILURE, FIRE DET MODULE SHOWS APU LOWER DETECTOR FAULT. REF B737-800 AMM 26-15-01, REMOVED/REPLACED APU LOWER OVERHEAT DETECTOR ASSY. OPS NML REF SAME. A/C CONFIGURATION RETURNED TO NORMAL. GROUND VERIFICATION TEST GOOD.