Car Heater Suddenly Not Working?

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Illustration of a person shivering in a car and saying Brrr...

Lifewire / Maddy Price

If you struggle with a car heater that never quite heats up enough to get you feeling toasty on the way to work, you're not alone. The good news is that automotive heating technology is pretty simple compared to a lot of the other systems in your car, as it hasn’t really changed a whole lot over the years. The bad news is that there are a lot of different things that can cause a heater to stop working. Let's explore a few things that could be wrong.

Two Key Categories of Car Heater Problems

Some of the problems that can cause a car heater to suddenly stop working are relatively easy fixes, while others will fall under the “take it to a professional mechanic” umbrella for most vehicle owners. In order to get a better idea of what happened, and what you might need to do to fix it, you’ll have to narrow things down a little.

For instance, a heater that blows cold could be a plugged heater core or low coolant, while a heater that doesn’t blow at all could indicate a bad blower motor or switch. In any case, it's best to either fix the problem or explore alternative heater options, before the temperatures hit sub-zero levels.

Most car heater problems can be broken into two main categories:

  1. Car heaters that blow cold air.
    1. Something is usually preventing hot antifreeze from circulating through the heater core.
    2. The problem could be a switch or valve, or the heater core could be plugged.
    3. Start by checking the coolant level when the engine is totally cold, and fill it if it's low.
  2. Car heaters that don't blow at all.
    1. When a car heater doesn't blow at all, the problem is either a bad blower motor or something preventing power from getting to the blower.
    2. If the blower motor is getting power, it's probably a bad blower. If it isn't getting power, suspect the fuse, switch or blower resistor.
    3. To diagnose this, you may have to crack open the heater box or even remove part of the dash to get at the blower.

If Your Car Heater Suddenly Started to Blow Cold

First of all, it’s important to understand the basic way that most car heaters work. There are exceptions, but vehicles that use water-cooled engines also use the (extremely hot) coolant to warm up the cabin. This is accomplished by pumping the coolant through a component called a heater core, which is similar in design and functionality to a small radiator.

When a blower motor forces air through the heater core and the air passes into the cabin of the vehicle, the interior of the vehicle warms up.

If you’re dealing with a situation where cold air comes out of your vents, even when the vehicle is warmed up and you’ve set the thermostat to hot, then you could be dealing with problems like:

  • Low coolant
  • Blockage in the heater core
  • A stuck blend door
  • A stuck heater valve
  • A bad switch or linkage

With the vehicle off, and after waiting long enough for the engine to cool down and the cooling system to depressurize, you can start by checking the coolant level. If the coolant level is low, then it may not be circulating enough hot antifreeze through your heater core to provide heat.

Filling the coolant may fix your problem in the short term, but low coolant often indicates another problem, like a leaking gasket or hose. In a worst case scenario, you may even be burning coolant, which indicates a blown head gasket.

There's a good chance that something, somewhere, has corroded and started to leak if a refractometer or other tester shows the pH is off. Coolant that isn't the right color or doesn't smell right also indicates a problem.

If your cooling system is full, then you can start the engine and allow it to warm up, and then check the temperature where the heater core hoses enter the heater core box — if it is located in the engine compartment — or pass through the firewall.

The safest way to do this is with a non-contact infrared thermometer. If one hose is the same temperature as the rest of the coolant, while the other hose is cold, then there is probably a blockage in the heater core. If the vehicle uses a valve in one of the heater hoses, it’s also important to check its operation. If the valve is stuck, and preventing coolant from flowing through the heater core, that is probably the source of your problem.

If you are able to determine that hot antifreeze is flowing through your heater core, then you may be dealing with debris in the heater box — typically in the form of pine needles and other detritus — or a blend door that isn’t moving.

If you switch the thermostat from hot to cold, and you can’t hear the blend door moving, that may indicate a problem with the blend door, linkage, wiring, or the thermostat switch, depending on your vehicle.

If Your Car Heater Doesn’t Blow at All

The other main way that car heaters malfunction, other than blowing cold air, is to not blow at all. This is typically due to a bad blower motor, but it can also be caused by a handful of other related components.

The only way to determine which component has failed is to grab some basic diagnostic tools, access the blower motor, and check whether it is receiving power or not. The blower resistor may also be bad, or the relay, or the switch itself. The specific diagnostic procedure will vary depending on your particular vehicle.

If you are able to determine that your blower motor is receiving power, then it is probably burnt out. However, there are rare cases where you may remove the blower and find that the squirrel cage is packed so full of debris that the motor is unable to operate. In other cases, you may find a broken wire, rusted connection, or even a pigtail that has become disconnected.

If, on the other hand, the blower isn’t receiving power, you will have to trace the problem back to the source, by testing the resistor, relay, and switch, although you may want to start by checking the blower fuse. A blown fuse often indicates a different underlying problem, so you should never replace one with a larger fuse to prevent it from blowing. However, if you replace a popped blower fuse with the same amperage fuse and it doesn’t go out again, the fuse may have simply failed due to age.