Why does the car heater suddenly stop blowing hot air?
2 Answers
During repairs, it is necessary to first determine the cause before proceeding with the corresponding repairs. The reasons for the car heater not blowing hot air are as follows: Thermostat stuck open or opening too early: This causes the cooling system to enter the major circulation prematurely. When the external temperature is very low, especially when the car is moving, the cold air quickly cools the antifreeze, preventing the engine water temperature from rising, and the heater will not blow hot air. Water pump impeller damage or slippage: This results in insufficient flow through the heater core, preventing the heat from rising. Air lock in the engine cooling system: An air lock causes poor circulation in the cooling system, leading to high water temperature and the heater not blowing hot air. If there is always air in the cooling system, it is likely due to a damaged cylinder head gasket allowing air into the cooling system. If the inlet pipe of the heater core is very hot while the outlet pipe is relatively cold, this indicates a blockage in the heater core, and the heater core should be replaced.
Just went through this, took me ages to figure it out. After driving for half an hour, the heater was still blowing cold air. Touched the vents on both sides of the dashboard and noticed a huge temperature difference—scorching hot on the left, icy cold on the right. Definitely a clogged heater core. Older cars are especially prone to this as debris in the coolant slowly builds up and blocks the passages. Also, don’t forget to check the heater hoses—if the engine is hot but the hoses are still cold, chances are the thermostat is stuck open. That little thing controls the valve for coolant flow into the heater core. The worst is a failing water pump—the dashboard shows normal coolant temperature, but the heater stays cold because the coolant isn’t circulating at all. Last time at the shop, I saw an even sneakier issue: mixed coolant types had created a gel that clogged the entire heating system into a cement-like mess.
