When Your Car Heater Blows Cold Air

Blown car heater
Tuomo Vainamo / Folio Images / Getty

There are a lot of different ways a car heater can fail, but the problem you’re describing is usually caused by one of two basic problems. Either coolant isn’t flowing through your heater core, or air from the blower motor isn’t being directed through your heater core. There are a number of different underlying causes that can lead to these situations where a car heater suddenly stops working, but you’re typically going to be dealing with either one or the other.

Quick Crash Course in Car Heater Operation

First off, everything here applies to vehicles with water cooled engines. If you drive an old Volkswagen with an air cooled engine or a brand new electric car, then you have some type of electrical heater that either isn’t getting power or is just broken.

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

This is the reason that it takes a while for heaters to start blowing warm air. Until the engine warms up, there’s no heat for the heater core to extract. It’s also the reason that a plugged heater core, stuck thermostat, or air in the cooling system can all cause a car’s heater to blow cold.

Car Heater Blowing Cold Due to Cooling System Issue

There are four main cooling system problems that can cause a heater to blow cold:

  1. Stuck thermostat
  2. Air in the cooling system
  3. Plugged heater core
  4. Coolant not flowing through the heater core

It’s a little more complicated than that in practice, but these are the most common issues that you’ll run into.

Thermostats are essentially valves that open and close depending on the temperature of the coolant. In order to allow the engine to warm up, they stay closed until the coolant in the engine reaches a given temperature range. And if they fail to open at that point, then coolant won’t circulate properly, the engine may overheat, and you might experience a problem where the heater blows cold.

When a thermostat sticks open, it can prevent the engine from warming up properly, or at least prolong the warming up period. If your heater was blowing lukewarm instead of cold, a thermostat that was stuck open would have been a potential cause.

Another common problem is when air gets into the cooling system. Since the heater core is often the high point in a cooling system, air can move into it and become trapped. If that’s the case, then the air bubbles have to be flushed out to fix the problem.

Plugged heater cores can also cause a car’s heater to blow cold. The best way to check for this is a non-contact thermometer, which will allow you to check if coolant is flowing through the heater core or not. If it isn’t, then flushing the heater core will often fix the problem.

Some vehicles have a valve installed in the heater core inlet line that is operated by vacuum or a mechanical cable. If that valve is stuck closed, then that’s another reason a car heater will blow cold.

Finally, a heater core can become plugged in more than one way. When you hear about a plugged heater core, what that usually means is that corrosion and other junk has clogged up the internal tubes, and flushing will often clear it up. However, the fins of a heater core can also get clogged up with lint, pine needles, and other detritus that manages to get into the heater box. The fix for this, of course, is to break open or remove the heater box and clean the fins.

Other Reasons a Car Heater Can Blow Cold

Most of the reasons that a car heater will blow cold have to do with the heater core, but you can also have a mechanical, electrical or vacuum problem. The specifics will vary quite a deal from one vehicle to another, but most systems have some kind of blend door that changes how air flows or doesn’t flow, through the heater core.

When a blend door gets stuck, it doesn’t matter if the heater core is working perfectly fine. Since the blend door is stuck, the heater core is essentially bypassed, and you won’t get anything but cold air.

Of course, there are a number of reasons a blend door can stick, and they don’t always stick the same way. A blend door can get stuck open, resulting in all heat all the time, or stuck partially closed so that all you get is lukewarm heat.

A blend door can also get stuck because of mechanical linkage or a vacuum line coming off, a switch going bad, or a number of other reasons. If you suspect that you’re dealing with a blend door issue, the specific diagnostic procedure will depend on how your vehicle’s heating system is set up.