The function is to automatically adjust the amount of water entering the radiator according to the temperature of cooling water, and change the circulating range of water, so as to adjust the cooling capacity of the cooling system and ensure the engine to work in a suitable temperature range. The thermostat must be kept in good technical condition, otherwise it will seriously affect the normal work of the engine. If the thermostat main valve is opened too late, it will lead to overheating of the engine; If the main valve is opened too early, the engine warm-up time will be prolonged and the engine temperature will be too low. At this time, it can be judged whether the thermostat is in good working condition. When the engine starts to run cold, if there is still cooling water flowing out of the water tank inlet pipe, it means that the thermostat main valve cannot be closed. When the engine cooling water temperature exceeds 70 degrees Celsius, there is no cooling water flowing out of the water tank inlet pipe, indicating that the thermostat main valve cannot be opened normally and needs maintenance.