There are many different reasons why a car heater can fail, but the problem you describe is usually caused by one of two main problems. Either coolant isn’t flowing through your heater core, or air from the fan motor isn’t being directed through your heater core. There are a number of different underlying causes that can lead to situations where car heater suddenly stops working but you will usually deal with one or the other.

A man in a car drives through the snow
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Quick Crash Course on Car Heater Operation

First, everything here applies to cars with water-cooled engines. Whether you drive an old Volkswagen with an air-cooled engine or a brand new electric car, then you have some type of electric heater, which either does not receive power or simply fails.

Most cars on the road still have water-cooled engines and all of their heating systems work on the same basic principle. Hot coolant from the engine passes through the heater core, which looks and functions much like a small radiator, and the fan motor forces air through it. The air is then heated by the coolant and then, in turn, heats the interior of the car.

This is the reason why the heaters start blowing warm air. Until the engine warms up, there is no heat to extract the heater core. This is also the reason why a clogged heater core, a stuck thermostat, or air in the cooling system can cause the car heater to run cold.

Car heater is blowing due to problems with the cooling system

There are four main problems with the cooling system that can cause the heater to overheat:

  1. Stuck thermostat.
  2. Air in the cooling system.
  3. Clogged core heater.
  4. Coolant does not flow through the heater core.

This is a little more difficult than in practice, but it most common heater problems, with which you will encounter.

Thermostats are valves that open and close based on the temperature of the coolant. To allow the engine to warm up, they remain closed until the coolant in the engine reaches a predetermined temperature range. And if they don’t open at this point, the coolant won’t circulate properly, the engine may overheat and you may have a problem with the heater blowing cold.

Opening the thermostat may interfere proper engine warm-up or at least extend the warm-up period. If your heater was blowing a little warmer instead of cold, the potential cause would be the thermostat opening.

Another common problem is when air gets into the cooling system. Since the heater core is often the highest point in the cooling system, air can enter and become trapped. If this is the case, the air bubbles must be removed to correct the problem.

Clogged heater cores can also cause a cold car heater. The best way to check this is with a non-contact thermometer which will allow you to check if coolant is leaking through the heater core or not. If this is not the case, then flushing the heater core will often solve the problem.

In some vehicles, a valve is installed in the inlet line of the heater core, which is actuated by vacuum or a mechanical cable. If this valve closes, then this is another reason why the car heater will blow cold.

Finally, the heater core can be connected in more than one way. When you hear about a clogged heater core, it usually means that corrosion and other unwanted substances are clogging the inner tubes, and flushing often cleans them up. However, the heater core fins can also become clogged with lint, pine needles, and other detritus that can enter the heater box. The solution to this, of course, is to hack or remove the heater box and clean out the fins.

Other Reasons Your Car Heater Might Blow Cold

Most of the reasons a car heater will be cold is due to its core, but you can also have mechanical, electrical, or vacuum problems. The specifics will vary greatly from one vehicle to another, but most systems have some sort of mixed door that changes how air flows or doesn’t flow through the heater core.

When the faucet door gets stuck, it doesn’t matter if the heater core works perfectly. Since the faucet door is stuck, the heater core is practically bypassed and you get nothing but cold air.

Of course, there are a number of reasons why a mix door can stick and they don’t always hold the same. The mixing door can get stuck open resulting in constant heat, or close partially closed so all you get is lukewarm heat.

The faucet door can also get stuck due to a mechanical clutch or disconnected vacuum line, a switch failure, or a number of other reasons. If you suspect you are dealing with a mixed door problem, the specific diagnostic procedure will depend on how configured heating system your his car .

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