Thermostat: It regulates the engine's temperature and it changes the refrigerant flow into the radiator.
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It is activated automatically due to the temperature of the refrigerant that flows into it. If the temperature is low, it will be closed and it will stop the refrigerant from flowing into the radiator, and this will keep the engine warm (during the ignition process or in really cold days). If it is hot, it will open and let the refrigerant flow through the radiator.