Passat warm air is not hot. What happened?

Passat heater is not hot, which may be the problem of air flow control mechanism or cooling system. The following two problems are introduced in detail: the possible problems of the cooling system: the thermostat is often turned on or turned on too early, which causes the cooling system to break prematurely or lose the impeller of the large circulating water pump, so that the flow through the warm air small water tank is not enough and the heat cannot come up. There is air resistance in the engine cooling system, which leads to poor circulation of the cooling system, high water temperature and not hot warm air. Problem of air volume control mechanism: If the air volume is insufficient or the distribution of cold and hot air is uneven, the heat of the small water tank of the warm air cannot be dissipated, and the temperature of the warm air will not rise. Check whether the hot and cold air control flap cable in the heater box falls off, whether the heater impeller is damaged, whether the flap falls off, etc. After troubleshooting, the heater will heat up.