HouseCallAuto 5/15/2010In a dual system there are two blend doors which mix cold air with heated air to achieve the temperature called for on that side. Each blend door has an electric actuator (gear motor). The actuator on the driver side is stuck or no good. In either case it means replacing it. It is not hard to replace the left side actuator but you should always rely on a repair manual that has the steps for this repair within it.