A thermostat responds to changes in temperatures. If the thermostat is stuck in an open or closed position, it will not regulate the temperature and will need replacing. The thermostat works by the opening or closing a valve to control coolant flow between the radiator and engine. The valve itself operates through a wax-container element. Therefore, the thermostat is small, simple and effective. The escalating component in the thermostat faces and links the engine coolant. As the coolant temperature increases, the fluid (or wax) container melts and enlarges, pushing a small bar that splits a centre plate from its neighbouring mounting base to expose the valve. It is essential that your thermostat is in working order to maintain a fully functioning engine.