Why is the air conditioner not blowing hot air?

[SanGabriel]

Here are the reasons why a car's air conditioner fails to blow hot air: 1. Insufficient coolant in the cooling system. The car's heating cycle involves a parallel circuit in the engine coolant's small circulation loop, which connects to the heater core. The blower blows air over the heater core to generate warm air, which is how the air conditioner produces heat. A lack of coolant in the cooling system is the most common and easily fixable issue causing the heater to not produce warm air. 2. Thermostat malfunction. If the thermostat is stuck open or opens too early, the cooling system prematurely enters the large circulation mode. In cold external temperatures, especially when the car is moving, the cold air quickly cools the antifreeze, preventing the engine water temperature from rising and thus the heater from producing warm air. 3. Heater core blockage. The heater core is essentially a scaled-down version of the engine radiator, consisting of multiple small cooling tubes. Like the radiator, it is prone to internal blockages. When the heater core is blocked, the flow rate and volume of coolant passing through it decrease, resulting in insufficient heat and thus no warm air from the air conditioner. 4. Heater valve failure. The heater valve is installed between the heater core inlet pipe and the engine's small circulation loop. It is controlled by the air conditioner's computer to adjust the valve opening, thereby regulating the coolant flow into the heater core for more precise control of the air conditioner's heating temperature. 5. Clogged cabin air filter. If the cabin air filter is not cleaned for a long time, dust accumulation on the filter can impair the heating efficiency.