A Thermostat is the little valve that controls coolant flow. When the engine is cold, it stays shut so the engine warms up quickly. Once it hits the right temperature, the valve opens and lets coolant flow to the radiator to cool down. We install it between the engine and radiator. It’s small but crucial – if it sticks shut, the engine cooks itself, almost like forgetting to open a window on a hot day. We replace it whenever it gets sticky.