As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FAULT LIGHT ILLUMINATED ON START ATTEMPT (ON GROUND TAXING IN PORTLAND) NO APU START. GROUNDED - YES. REMOVED & REPLACED APU FUEL CONTROL UNIT & LUBE UNIT REF B737-900 AMM 49-31-11 & 49-91-11. STARTED & RAN APU MULTIPLE TIMES WITH NO ISSUES NOTED. GROUND VERIFIED APU OPS GOOD.