What was found
AFTER GEAR RETRACTION ON DEPARTURE, LEFT GREEN GEAR DOWN INDICATION REMAINED ILLUMINATED. DID APPROPRIATE E AND A CHECKLIST.ALSO VISUALLY CONFIRMED THAT GEAR WAS RETRACTED. ALSO VERIFIED WITH ALTERNATE GEAR DOWN LIGHTS ON FLOOR THAT ALL GEAR WERE RETRACTED. AFTER SWITCHING CAUTION / ADVISORY LIGHT SWITCH FROM DIM TO BRIGHT THE LEFT GREEN GEAR DOWN LIGHT EXTINGUISHED. WITH CAUTION / ADVISORY LIGHT SWITCH IN DIM, LEFT GEAR INDICATES DOWN, WITH SWITCH ON BRIGHT ALL GEAR SHOWED RETRACTED. ALSO DID ALTERNATE GEAR EXTENSION CHECKLIST. REPLACED ADVISORY CONTROL UNIT. FOLLOWING REPLACEMENT OF ACU, THE LH DOWNLOCK SENSOR WAS "CHEATED" TO SIMULATE UNLOCKED CONDITION AND CORRECT INDICATION WAS PRESENT IN DIM AND BRIGHT MODES. CHEATER REMOVED AND ALL INDICATIONS RETURNED TO DOWN AND LOCKED. RETURNED TO SERVICE.