The car's engine thermostat allows the passage of hot coolant to the radiator, so that the heat generated in the engine is eliminated.
You can test the thermostat by putting it in boiling water for about 20 seconds, if the thermostat is ok then the valve will open.
If you take the thermostat out of the boiling water and wait a few seconds, the valve will close when the temperature drops.
This test is valid for altitudes up to about 3000 meters above sea level, because as the altitude rises, the water boils at less than 100 degrees Celsius, even reaching about 85 degrees, and it would be the limit to open the thermostat that begins its opening at 82 degrees Celsius. .