What was found
ON APPROACH AFTER LANDING GEAR SELECTED DOWN, THE LEFT MAIN GEAR UNLOCKED INDICATION MESSAGE WAS DISPLAYED. CREW RECYCLED LANDING GEAR. DECLARED AN EMERGENCY AND PERFORMED A GRAVITY EXTENSION RESULTING IN ALL THREE GEAR INDICATING DOWN AND LOCKED. AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED LEFT MAIN DOWNLOCK SENSOR 15GA, OPERATIONAL CHECK OK.