As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PER FLIGHT CREW REPORT APU WOULD NOT START FAULT LIGHT ILLUMINATED. GROUNDED - YES. PERFORMED APU BITE PROCEDURE REF B737-900 FIM 49-60 TASK 801 FOUND MX MESSAGE 49-41184 IN CURRENT STATUS. COMPLIED W/ FIM 49-40 TASK 804 THRU STEP F(4) NO ISSUES FOUND W/ WIRING HARNESS. C/W VISUAL INSPECTION OF TAIL CONE AREA NO DEFECTS NOTED. REMOVED AND REPLACED APU IGNITION UNIT IAW B737-900 AMM 49-41-31. APU OPS CHECK OK.