Yamaha Thermostat Switch YF1-75388-30-40. Overview. The Yamaha YF1-75388-30-40 Thermostat Switch is a genuine original equipment manufacturer component designed to help monitor and regulate engine temperature within the cooling system. This switch works alongside the thermostat and cooling components to help detect temperature changes and support proper engine operation.. Maintaining the correct engine temperature is critical for efficiency, performance, and long term durability. A properly func