As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NR 1 GENERATOR CONTROL UNIT SHOWS "GCU FAIL" FAULT LIGHT AND WILL NOT RESPOND TO BITE. REMOVED AND REPLACED GCU 1. ETOPS ROTABLE COMPONENT VERIFICATION. C/B P6 4 F10 CLOSED. PERFORMED GCU INSTALLATION TEST AND OPERATIONAL TEST OF AC GENERATION AND CONTROL SYSTEM ALL TESTS PASS.