As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) RETURNED DUE TO AN ELECTRICAL PROBLEM (RT DC BUSS FAULT). ENGINE 1 THRU 4 GROUND RUN. DC SYSTEM BROUGHT ONLINE. NR 4 DC GEN OPENS DC BUS TIE AND AFTER A SHORT DELAY THERE WAS A RIGHT DC BUS FAULT. REPLACED NR 4 DC GCU. SYSTEM FUNCTION CHECKED, NO FAULTS FOUND.