What was found
CREW RECEIVED EICAS MSG FOR LEFT AC BUS TIE FAILED, AND STATUS MSG SHOWING BOTH LEFT AND RIGHT FAILED. *S/D* ELEC BUSS ISOLATION MSG L AND R EICAS. RIGHT BUSS TIE ISOLATION LIGHT ONLY OVERHEAD. RESTORED ELMS EU'S, RAN BOTH ENGINES AND TRANSFERRED PWR BETWEEN ENG GENERATORS, APU AND EXTERNAL PWR, LEFT & RIGHT BUSS TIES OPERATE NORMALLY, PER MAINT TIPS. AIRCRAFT OK FOR SERVICE. ENGRNG REQUESTED UNIT REMOVAL. KNOWN PROB WITH GCU & EXISTING SERVICE BULLETIN CORRECTS. MOD DONE BY ATTRITION.