As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
"ELECTRIC" LIGHT AND MASTER CAUTION ILLUMINATED AFTER SWITCHING POWER SOURCE FROM EXTERNAL TO APU. ACFT GROUNDED. PERFORMED FIM TASK 24-31, TASK 801, P5-13 PANEL BITE NORMAL WITHOUT FAULTS. FOUND APU GCU UNABLE TO BITE. RERACKED APU GCU. PERFORMED FIM TASK 24-21, TASK 801. NO FAULTS FOUND.