As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
TRANSFER OF POWER AFTER LANDING TO APU NR 1 TRANSFER BUS OFF LIGHT CAME ON. GROUNDED - YES. ACCOMPLISHED TRANSFER BUS OFF LIGHT ILLUMINATION FIM TASK 24-21-00-810 FOUND BTB FAULT AT NR 1 GENERATOR CONTROL UNIT. REMOVED AND REPLACED GCU 1 REF AMM 24-21-81. OPS CHECK GOOD. REPLACED NR 1 BUS TIE BREAKER REF AMM 24-21-41. OPS CHECK GOOD.