Toyota Aygo Thermostats Available Here. Car thermostats are an important part of your cooling system. The thermostat ensures the engine gets up to temperature quickly which limits wear and means your interior heater will warm up quickly. The job of the thermostat is then to help regulate temperature of the coolant. Thermostats typically respond to changes in temperature by opening or closing a valve to regulate coolant flow between the engine and the radiator. Drive with a failed thermostat that is stuck in the closed position and your engine could overheat causing serious damage. If your thermostat is stuck in the open position your engine may take forever to warm up which casues increased wear, increased emissions and reduced fuel economy.