As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING APU START, AC FLAGGED A APU FAULT UNABLE TO ACCEPT A/C POWER (GROUND). GROUNDED - YES. REMOVED AND REPLACED AUX POWER BREAKER, C803 IN P91 POWER PANEL. OPS TEST GOOD; APU START WITH GEN TRANSFER SUCCESSFUL. REF B737-9 AMM 24-21-41. A/C EXT POWER OPS TEST GOOD. REF AMM 24-41-00. GROUND VERIFICATION TESTS GOOD.