CARS.COM — Why does your car heater blow cold air? Chances are that if you’re getting a blast of cold air you don’t want when you turn on the vehicle’s heater or defroster, it’s a sign of the same problem: Something has gone wrong in or around the heater core — a small radiator through which a fan blows air into the cabin to keep everyone toasty.
The core, a type of heat exchanger, gets its heat from liquid engine coolant that circulates through it by means of heater hoses. Air driven by the blower picks up the heat on its way into the passenger compartment. Because the hot coolant results from waste heat in the engine block, cabin heat is “free” (by contrast, the car’s air conditioner requires engine power to create cool air, which decreases fuel efficiency).
Because the heater core is usually in an inconvenient location, such as behind the dashboard, and might require the dash to be partially removed just to access it, you might want to have a pro look elsewhere first to eliminate simpler possibilities.
First, check the coolant level when the engine is cold. If it’s really low, it might not be able to warm the heater core, even if it’s doing an adequate job of preventing engine overheating. The cooling system thermostat also may be failing to open, preventing coolant from circulating. In the latter case, there’s a good chance your engine will be running hot and be in danger of overheating.
If neither of those is the cause, then the heater core and ancillary components are likely culprits. If hot coolant isn’t circulating through the heater core, perhaps a valve isn’t opening to allow that flow, or the core itself is leaking or clogged, or perhaps developed an air lock if air wasn’t properly bled following cooling system maintenance. (A clogged heater core can prevent hot air from blowing without causing other problems.) Another possibility is that a door or diverter that is supposed to direct warm air into the interior is stuck, preventing proper airflow.
On cars with automatic temperature control, a “set-and-forget” feature that is supposed to maintain a constant temperature, the computer that controls the system might be malfunctioning. It’s possible that the heater core and other parts are fine, but the computer has grown tired of constantly catering to your climate needs and stopped answering your calls.
Because diagnosing and fixing these kinds of problems can be complex, most consumers would likely be better off seeking professional assistance for these particular auto repairs. If you search the internet, you will find many instances in which do-it-yourselfers say they did this and tried that, but their heater still blows cold air.
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