As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE TOWING A/C APU FAULT LIGHT CAME ON AND APU UNABLE TO START. GROUNDED - YES. PERFORMED APU BITE FOUND FAULT 49-31175 WAS 75 STARTS OLD AND NOT LIKELY RELATED TO CURRENT PROBLEM. INSPECTED APU WIRE HARNESS AND RANG PINS ON FCU PER B737-800 FIM 49-32 TASK 805 WITH NO DEFECTS NOTED. STARTED APU MULTIPLE TIMES WITH NO FAULTS.