What was found
ELEC GEN SYS L STATUS MESSAGE. FOLLOWED BY GALLEY LOAD SHED INDICATIONS. BUS TIE SWITCH DID NOT INDICATE ISOLATED HOWEVER NO CONNECTION ON SYNOPTIC PAGE TO BUS TIE. APU GEN & BUG ALSO DID NOT INDICATE IT WAS POWERING THE L BUS. CREW CYCLED L GEN SWITCH TO OFF THEN BACK ON AND NML IND RET. BOEING AHM DETECTED FAULT MSG 24-38801-GCU L HAS NO OUTPUT ON ANY BUS. REPLACED NR 1 GCU, INSTALLED NEW SOFTWARE, RAN ENGINE, ALL OKAY.