There are four possibilities for the car heater not being hot: The first type of vehicle cooling system lacks a certain amount of coolant, causing the cooling water to not circulate normally in the heater water tank, resulting in the phenomenon that the heater is not hot. , the second type is that the coolant in the vehicle is not replaced in time according to the three-year and 60,000-kilometer regulations, resulting in the water channel of the heater tank being blocked, the coolant cannot circulate normally, and the heater is not hot. The third type is because the air-conditioning filter element is not replaced in time. Excessive dust and impurities on the surface of the air conditioning filter affect the air inlet volume, resulting in too little air output from the air outlet and the warm air is not hot. The fourth type is excessive dust and impurities on the surface of the heater water tank, which affects the air flow and results in the warm air not being hot. Among them, the third and fourth situations that are most likely to cause the heater to not heat up are likely to occur. The fourth situation needs to occur in vehicles without air conditioning filters. It is recommended that the vehicle coolant should be replaced every three years and 60,000 kilometers, and the air conditioning filter element should be inspected every 10,000 kilometers and replaced every 20,000 kilometers.