1. Let's see if the engine water temperature is normal. If it has been in a low temperature state and does not reach the working temperature, the warm air will definitely not be hot.
2. Focus on whether there is a large temperature difference between the inlet and outlet pipes in front of the heater box. If this happens, the heater box must be blocked.
3. It depends on whether the flow rate and temperature of coolant entering the warm air water tank meet the requirements.
It depends on whether there is air resistance in the cooling system. It is common that the coolant is not used up after replacement.