As complex as modern cars are, one of their most useful components is surprisingly simple. Car heaters are little more than a small radiator-like device (called a heater core) through which coolant runs. A fan blows the air over the coolant that has captured engine heat, helping to heat the interior cabin of the car.
While this system is quite simple, there are unfortunately a wide number of things that can go wrong with a car’s heating system. When problems strike, it’s likely that the interior of the car won’t get enough heat to keep you and your passengers comfortable. This is especially true in the winter months throughout most of the United States, when a malfunctioning heating system can even become dangerous.
Luckily, most problems with automotive heating systems are caused by a few predictable things. Let’s take a look at several possible reasons why your car heater may be blowing cold air.
Because your car’s heater core captures engine heat through the coolant that flows through the engine compartment, when coolant levels begin to drop below recommended levels, the amount of heat transferred from the heater core to the interior of the car begins to rapidly diminish. Once coolant levels fall below a certain level, you’ll no longer be able to feel any heat coming through the vents.
Many things can contribute to low coolant levels. The most common cause is simply not keeping the coolant topped up. Coolant can leak and evaporate through hairline cracks and tiny imperfections in seals and gaskets.
You may not even know these defects in your car’s cooling system exist. But as cars age, cracks in hoses, caps that don’t seal, and slowly degrading gaskets will naturally begin to cause coolant leaks. For that reason, it’s important to keep your coolant levels topped up, especially in winter.
Another thing to watch out for is an improper coolant-water mixture. In certain regions in the US that experience extreme cold, even a little bit of water in the cooling system may be enough to form ice. If ice blocks the flow of engine coolant, the heater core may not provide any heat at all.
Coolant lines blocked by ice make for a particularly dangerous situation as it can result in your car’s engine overheating in minutes. Any time your car has been running for a while and ice-cold air is still coming out of the heating vents, make sure to check that your engine is not overheating.
Compromised heater core or blower motor
A blocked heater core or a blower motor that is malfunctioning can cause a lack of heat generation. Heater cores are extremely simple parts, being really nothing more than a smaller version of your car’s radiator. But over time, the small tubes within them can become clogged, preventing coolant from flowing through.
Blower motors contain moving parts that are subject to wear like any other car part. If you have an older vehicle, your blower motor may eventually fail. You might be able to tell that a blower motor is causing the problem by noticing reduced or absent air flow.
Clogged or broken heater controls or valves
The controls and valves within your car’s heating system get so much use, so it’s likely that they’ll eventually break or become compromised. Heating controls that stop working may be obvious in cars with electronic climate control systems. But in cars with analog controls,you may have to isolate the problem.
Another common issue that can cause ice-cold airflow is the heater control valve becoming stuck or losing mobility. This is also an issue that’s all but inevitable in older cars. A bad heater control valve may be discovered by ruling out other causes of cold air or, in some cases, by direct observation.
A broken thermostat
Lastly, a bad thermostat may be the single most common cause of heating systems blowing cold air. Thermostats rely on the expansion and contraction of metal coils or springs to relay temperature information. When they go bad, as they inevitably will, it may result in cold air being produced by the heater.