If you have a car heater blowing cold air and no heat, here’s five common reasons why this can happen. It could be a cable or electrical issue, or you could have a problem with the cooling system, like a blockage or leak.
Top 5 Causes of a Car Heater Blowing Cold Air and No Heat
If your car heater blows cold air and no heat, there may be a problem with the switch, the cable leading from the switch to the blend door, the blend door actuator, or the cooling system.
1. Defective Switch
You’ll have a switch that lets you change the temperature range in your car from cold to hot. If you still have cold air blowing by the time you should have heat, you’ll usually have a problem with the switch’s cable. The internals of cars with electrical switches like the circuit board can break it if dirt, debris, and moisture collect inside.
If the cable leading to the blend door actuator is broken, frayed, or crimped, that can restrict the movement of air flow and cause cold air to blow through the vents when you have the settings on for hot air. If the cable’s outer sheathing disconnects from its mounting point, that can also affect the blend door actuator’s ability to function.
2. Defective Cable
The heater switch sends a signal down the wiring to the blend door actuator, which is attached to the heater core box. The blend door actuator has internal components like a circuit board and motor that can lose its ability to function over time and fail to get the blend door actuator working.
3. Defective Blend Door Actuator
The blend door actuator sends air through the cold or warm sections of the heater box depending on the temperature setting. The heater in your car could be blowing cold air instead of hot air if the blend door actuator is broken and stuck diverting air through the cold section of the heater box.
4. Problem with the Cooling System
Clogged Cooling System
The engine gets hot as it burns fuel. The cooling system cools this down, and it also lets you warm up your car’s interior when you need to. As the coolant absorbs the heat from the engine, before it cycles through the cooling system and cools down, it travels through the heater core. The heater core sits behind the dash. The blower motor sends heat emitted from the heater core into the cabin when you turn the fan and heat on.
Cool air can also flow through the vents with the heat is turned on if there’s a problem with the heater core, like clogging.
To check the heater core for clogging, check the temperature gauge on the dashboard. If you ran the vehicle for five to ten minutes and the temperature gauge is still reading as cold, your cooling system may have a clog or a leak.
A dislodged or crimped hose can cause blockage in the cooling system and restrict the flow of coolant. Using sealants to plug leaks in the cooling system can also cause blockages. The remaining particles of the sealant used to stop the leak can float around in the cooling system and restrict flow.
Note: We recommend only putting manufacturer-specified coolant into the cooling system.
Coolant System Is Leaking
To check for a leak, you’ll want to wear hand and eye protection and open the hood. Then with the cooling system cooled down and the radiator cap cool, remove the radiator cap. Check the coolant level. If the level looks low, there could be an air bubble in the system restricting the flow of coolant. An air bubble can prevent the flow of coolant, which won’t heat up the heater core or send warm air into the cabin.
Leaks generally happen on the radiator, intake or head gasket. While the water pump typically has a weep hole, you can also find leaks at the bottom of it.
More on how to check the cooling system for leaks
5. Engine Didn’t Warm Up
Smaller engines like an inline 4-cylinder will take longer to warm up than a V8 engine, for example. The cooling system may not retain any heat if you’re driving a short distance. You can also have a heater blowing cold air if the engine hasn’t been running for too long.
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