Does the fan continue to run after it has met temperature? That is the first thing to check. After the fresh food thermistor ( on the walls) is satisfied ,the valve will shut off the refrigerant to the fresh food evaporator. The fan will run at low speed until the thermistor mounted on the evaporator is satisfied at 35 degrees. If the fan turns off too soon, it will frost up. Changing it is not that easy.