As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN 2 APU START ATTEMPTS, AS SOON AS SWITCH IS SELECTED TO START, APU FAULT LIGHT ILLUMINATES. GROUNDED - YES. CHECK FOUND MESSAGE NBR 31-01011 NO INPUT FROM APU ECU. REPLACED APU ECU, PERFORMED GOOD INTALLATION TEST REF B737-9 AMM 49-61-12/401.