As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FAULT LIGHT ILLUMINATES ON ATTEMPTED APU START. GROUNDED - YES. ACCOMPLISHED APU BITE TEST, FOUND ACTIVE MAINTENANCE MESSAGE 49-89002 " ONBOARD MAINTENANCE FUNCTION HAS NO INPUT FROM APU ECU" PER B737-9 IFIM 49-00-00-810-817, REMOVED AND REPLACED APU ELECTRONIC CONTROL UNIT REF B737-9 AMM 49-61-12/401. START APU AND PERFORMED APU BITE TEST, NO ACTIVE FAULT NOTED.