Why Does My Car Heater Blow Cold Air?

Say your engine is at its normal operating temperature, the heater is turned on, the temperature is set to something warm and cozy, and you can only feel cold air blowing out of the vents. If you’re asking for heat but only cold air is coming out, the problem likely lies in one of two areas: Either hot coolant isn’t circulating through the heater core behind the dashboard, or air isn’t being blown through the warm heater core and out the vents.
Related: Tips for Starting Your Car in Subzero Weather
However, sometimes this issue can be trickier to diagnose. If your car has automatic climate control where you only have to set a temperature and the car maintains that automatically, a computer makes adjustments that are otherwise adjusted manually, such as whether to turn the heater on or off. That computer itself can fail and may require calling in a professional.
What Are the Basics of In-Car Heating?
In most cars, if everything’s working as it should, the warm air blowing into the cabin when the heat is turned on comes courtesy of the car’s cooling system that keeps the engine from overheating. The engine produces a lot of heat when it’s running; to keep it from overheating, coolant (a mixture of water and antifreeze) is circulated through the engine and then to the radiator, which cools the coolant and sends it back to the engine to cycle through again.
To get heat into the cabin, some of that hot coolant is directed to what’s essentially a small radiator located behind your dashboard called the heater core. Hot coolant warms the heater core; a fan then blows air through the heater core to warm the air. That’s what produces the warm air that heats the cabin.
There are cars where this works a bit differently. Air-cooled cars don’t use coolant and rely on other means to heat the cabin, such as heat exchangers that send air over the hot exhaust, to warm the car’s interior. Likewise, heaters in electric vehicles don’t rely on an internal combustion engine to generate heat because they don’t have one, so unfortunately, troubleshooting those is also outside the scope of this guide.
Unfortunately, for something that seems so simple, there’s an awful lot involved in the process. Thus, an awful lot that can go wrong. Some heater components are hidden behind the dashboard, where they’re very difficult to reach; others are under the hood, which can be tightly packed on newer cars, likewise making things tough to access. You may ultimately decide that it’s best to leave many repairs and even diagnostics to a professional, but there are some things you can easily check yourself.

Some Simple Things to Check
Start from inside the car. Some of these suggestions might seem obvious, but it’s best to eliminate them as possibilities. First, make sure all of the climate controls are set to deliver heat, the temperature is set to either a high number or in the “hot” zone of your temperature dial, and air is blowing out of the vent. Someone may have simply changed a setting.
Next, be sure the engine is at or near normal operating temperature. Ideally, this is determined by your temperature gauge reading somewhere near the middle between full cold and full hot, and holding steady. (If the engine has been running for a while and the needle is near the bottom of its temperature range, you won’t get much — if any — heat, and there’s likely a problem with the cooling system that needs to be addressed. It may seem counterintuitive, but the same can be true if the temperature gauge is reading too hot.) If your car doesn’t have a gauge, be certain the engine has been running for several minutes and that no temperature warning light is on (this light would indicate the engine is overheating). Some cars may also have a blue coolant-temperature light that indicates when the engine hasn’t warmed up yet, so ensure this is off before you keep troubleshooting.
If all of that checks out, the next steps are under the hood.
First, check to see that there’s coolant in the coolant overflow tank. If not, shut off the engine, wait for the radiator to cool to the touch, remove the radiator cap, and check to see if the radiator is full of coolant. If not, turn off the car, let it cool down, then fill the radiator and add coolant to the “cold” line on the overflow tank. Low coolant can keep a car’s heater from working properly.
If that doesn’t solve the issue, start the car and get it back up to temperature, then feel the upper radiator hose that goes into the top of the radiator to see if it’s hot. Be careful, as this hose can be very hot. If it is, it’s an indication the engine is at normal operating temperature; if the hose is somewhat cool, it may be because the thermostat is stuck open. A stuck-open thermostat can prevent the coolant from ever getting hot, which in turn could result in no heat.
Here’s the big one: You can tell a lot if you can find the two heater hoses going to the heater core. These might be difficult to find in a jam-packed engine compartment, but the heater hoses can be black and about an inch in diameter, and they go from the engine into the firewall. These hoses are usually on the passenger side or toward the middle, and they’re typically fairly close together even if they’re not always the same length.
If you can find and reach them, check to see if both heater hoses as they enter the firewall are warm or hot. As with the radiator hose, be careful, as these hoses could be hot. If they’re both at least very warm — and they should be fairly close to the same temperature — that’s an indication that warm coolant is circulating through the heater core as it should.
If they aren’t both at least very warm, it means warm coolant isn’t circulating through the heater core. That may be because the heater core is plugged — or more likely, that there’s a heater control valve in the inlet hose that isn’t opening. Other culprits could include a low coolant level (which we already checked), broken serpentine belt, malfunctioning water pump or stuck-open thermostat, but those are unlikely to be the issue if the engine is running at normal temperatures.
Not all cars have a heater control valve, but if yours does, the valve is usually about the size of your fist, with a heater hose leading into it and another leading away from it into the firewall. The valve is typically opened via a cable, vacuum hose or electrical connection with wires. In some cases, you might be able to see something such as a lever or plunger that looks as though it would move to open the valve. If so, you may be able to move it manually to open the valve.
If the heater control valve is actuated by a cable or vacuum hose, that cable or hose not working as intended may be the problem rather than the valve itself.
If both hoses going to the heater core are at least quite warm, then the most likely reason why you’re not getting heat inside the car is a stuck blend door behind the dashboard. The blend door routes incoming air either through the heater core to get hot air or bypasses the heater core to get cold air. For warm air, it uses a variable combination of the two, hence the name “blend door.” If only cold air is blowing out of your vents when you have the controls set to get heat, it’s likely that the blend door is stuck in a position where it’s not routing all of the incoming air over the heater core.
Unfortunately, the blend door — being behind the dashboard and encased in a housing — is awkward to get to, and designs vary. However, you may be able to do an internet search for “How do I fix the blend door on a [year/make/model]?” In some cases, the blend door is moved by an electric motor that can be replaced, or you may be able to find a way to move the blend door manually.
If none of that isolates the problem or the likely issue is beyond what you would be able to fix on your own, it’s time to consult with a mechanic.
Heat in our cars is something we’ve come to take for granted, but there’s so much that makes it work, we should be grateful when it works.
More From Cars.com:
- Why Do My Tires Lose Pressure in Cold Weather?
- How to Survive Winter With Rear-Wheel Drive
- Should You Warm Up Your Car in Winter?
- How to Safely Remove Snow and Ice From Your Car
- Can a Car Battery Freeze?
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