As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
GROUND POWER DISCONNECTED, WOULD NOT TAKE APU POWER, EITHER TRANSFER BUSS OFF ILLUMINATED BOTH SIDES FOUND FAULT ON APU GENERATOR CONTROL UNIT FOR GCB/APB FAULT. CREW STATED: ELECTRICAL /RUBBER SMELL THROUGHTOUT CABIN AND FLIGHT DECK. APU GCU WILL NOT TEST FAULT REMAINS SUSPECT BAD APU GCU. R & R APU GCU IAW AMM 24-21-81-400-801. ACCOMPLISHED ALL GROUND TEST. RAN APU, ALL NORMAL. NO FAULTS, OP CHECKS NORMAL.