As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING APU START ON EXT PWR O/BS'D BATTERY DISCHARGE LT "ON". VIS INSP'D START CONVERTER & START POWER UNITS IN E & E. OBS'D APU START CONV UNIT C/B ON P91 115VAC TRANSFER BUS 1 OUT (A 11). REPLACED START POWER UNIT MM 49 41 71 000 801 & 400 801. APU START & OPS CHECK PASS MM TASK 49 41 00 860 801.