As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INSTALLED NEW GCU. APU GEN COMES ON LINE BUT ACFT DUMPS POWER WHEN SWITCHING BACK AND FORTH BETWEEN APU GEN AND GND POWER. NEW GCU STILL INSTALLED AND OLD GCU INSIDE OF EE COMPT WITH TAG FOR REINSTALLATION. REINSTALLED ORIGINAL APU GCU, RAN APU OPS CHECKS NORMAL. REPLACED GENERATOR, OP CHECKS GOOD.