What was found
LANDING GEAR DOOR OPEN LIGHT REMAINED ON AFTER GEAR WAS RETRACTED, RECYCLED GEAR WITH SAME RESULTS. EMERGENCY GEAR EXTENSION PROCEDURES WERE UTILIZED AND FLIGHT RETURNED TO ATL, LANDED WITHOUT INCIDENT. REPLACED LEFT MAIN INBOARD GEAR DOOR, RIGGED AND CYCLED DOORS SEVERAL TIMES. OPERATIONAL CHECK OK AND AIRCRAFT RETURNED TO SERVICE.