You can use this method to clean the water tank first, remove the inlet and outlet pipes of the warm air water tank, soak them in vinegar or acidic liquid for one hour, and then blow them clean with an air gun. It is also possible that the impeller of the water pump is damaged or lost, resulting in insufficient flow in the warm air box and the warm air is not hot, but the possibility is relatively small.
Extended data The warm air is hot when the fuel filler door is full, but it is not hot when the fuel filler door is not full. Check whether antifreeze is insufficient. The level of antifreeze should be in the middle of the scale. When the coolant level is too low, air may be sucked into the heater pipe, and there is air resistance in the engine cooling system. If the cooling system does not circulate, the warm air will not be hot. If there is air in the cooling system all the time, it may be that the engine cylinder pad is damaged, and the coolant needs to be replenished in time.
The lack of warm air in the car is mostly caused by insufficient cooling liquid or excessive impurities in the cooling liquid, and the pipeline is blocked. The coolant mixed with water caused the heater water tank to be blocked. So remember to check the coolant level during maintenance, replenish it if it is missing, and change it when it is time. Don't think that coolant is life-long.