The car’s heater is blowing cold air at idle. What are the causes?

A common problem that arises when driving in winter is that the air blown when the engine is idling becomes cold. There are a few reasons why the car’s heater gets cold at idle, which I will describe below.

If your car’s heater does not heat enough not only at idle, you could check the common reasons why the heater does not work or has problems with heating.

Related to that article the reasons why the heater blows cold air at idle are somehow similar with small differences.

5 reasons why the car’s heater blows cold air when the engine is idling

There are five main causes why the heater is cold at idle.

The low coolant level in the expansion tank

A low coolant level in the tank represents the main reason why you don’t have hot air in the cabin, it is not just related to when the engine is idling, however, this can be the reason why your heater blows cold air at idle.

Completing the coolant level with the same type of coolant represents an easy and quick fix. Also, if you see that coolant drops constantly without any signs of liquid leaks, check the radiator cap or the expansion tank cap.

You should not neglect this symptom because things can get serious and harmful to your engine. If the coolant has not been changed for a long time and/or is in poor condition, then it is better to replace it with a new one, the same brand, or with one that your engine needs, so it’s better to check the car manual.

A bad or failing water pump 

This part is responsible for the circulation of coolant through the cooling system, and when the rotor is not able to create sufficient flow, the heater blows cold air when the car is idling, and when the car is moving it may be slightly warmer. If the pump is responsible, then it needs to be changed to a new one.

A dirty or clogged heater radiator

The heater’s radiator clogs over time making it harder for the coolant to pass through its bearings, leading to slightly warm air or completely cold air blown at idle.

You can try cleaning the radiator and check if a crack has not been created where the liquid can come out or air can enter.

Usually, cleaning the heater’s radiator has a positive effect on the efficiency of the heater, including at idle speed.

A bad or worn thermostat

When the thermostat rod is jammed in the open position this can mean cold air is blown when the engine is idling. If the coolant is initially circulated in the “big circle” in a cold engine, it will only be able to warm up after a long time of driving the car, or you will need to spend a lot of time warming up when the engine is idling.

Problems with the climate control system

Especially on newer car models, this system can sometimes have software issues, which leads to cold air blown by the car’s heater at idle. 

Problems can be associated with improper configuration of a particular system, or with software or hardware problems of climate control.

Checking its operation depends on the specific car make and model and the type of system. These steps are usually described in the car manual. 

If there is such information, you can check it yourself. Otherwise, it is better to seek help from a car workshop.

Air in the cooling system

The presence of air in the cooling system can be due to improper replacement of the coolant, depressurization of individual pipes or their connection point, malfunction of the air valve, problems in the pump operation, or because of a damaged cylinder head gasket.

Air in the cooling system impedes the circulation of coolant in the system, and as a result, the heater blows hot air only when driving, and cold air at idle.

The coolant supply is blocked

If the heater has a tap for supplying coolant to the radiator, then this is closed or jammed because of rust. This type of heater is present, in most cases, on older cars and the simplest solution is to open the tap. When using more water than the coolant, rust will form which jams the tap.


If the car’s heater is blowing cold air at idle, you should first check the coolant level, and its condition, completing the coolant level is the easiest way but not always the best way to fix this issue.

Next, you need to check the water pump, thermostat, heater’s radiator, heater tap, and the presence of air in the cooling system.

You should remember that a poorly functioning heater, in any case, indicates a problem in the engine cooling system, and driving a car with these problems can lead to bigger problems and costly repairs in the future, so it’s safe to inspect the problem as soon as possible to avoid bigger issues.

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