As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU WOULD NOT START WITH FAULT LIGHT ON GROUND. WAITED FIVE MINUTES AND ATTEMPTED AGAIN, SAME RESULT. ACFT GROUNDED. ACCOMPLISHED FIM 49-40, TASK 805. AC & APU WIRING IS GOOD. NO DEFECTS NOTED. CHANGED APU IGNITION UNIT AS PRECAUTION IAW MM 49-41-31-400-801. OPS CHECKED GOOD.