The reasons are as follows
 1. The cooling system of the engine is blocked by air, which causes the cooling circulation system of the car to not operate normally, thus leading to the problem that the heating air is not hot. If the small water tank for heating air is hot in the inlet pipe, and the small water tank for cooling water in the outlet pipe is blocked, it needs to be replaced.
 2. The water pump impeller of the vehicle is damaged or lost, which reduces the amount of water flowing through the water tank, leading to the lack of heat of the warm air, and needs to be replaced with a new water pump impeller.
 3. It may also be that the thermostat is always opened or opened prematurely, which causes the cooling system to cycle ahead of time, and the external temperature of the vehicle is low, so the external cold air will rapidly cool the antifreeze, resulting in insufficient temperature rise in the water tank and insufficient heating of the warm air. The thermostat needs to be adjusted or repaired.