The engine coolant thermostat maintains the temperature of the coolant in the engine to a preset level. The internal combustion process makes a lot of heat, and this heat is dissipated by the radiator when hot coolant is pumped to it by the action of the water pump pushing it through a series of passages and coolant hoses. The thermostat is in one of those passages and does its work by being a temperature sensitive valve allowing hot water to the radiator when needed. Making sure that the engine stays at its proper temperature is essential to the engine performance, fuel economy and the longevity of the engine oil and, in the long run, the engine itself. The thermostat should be replaced on a schedule well before it is in danger of failing because they can get out of calibration as they age and total failure can cause catastrophic engine damage.