Car Heater Blowing Cold Air? Cooling-System Causes to Check Before Winter

When a heater blows cold air, the first clue is whether air is coming out of the vents at all. If it flows but stays cold after the engine has warmed up, the problem is on the heat side: not enough hot coolant is reaching the heater core, or the air isn't being sent across it. If little or no air comes out, the blower and its controls are the place to look.

Either way, have it checked before the cold sets in. If the coolant is low, that usually points to a leak or another fault that a cooling system check should track down. Fully electric vehicles heat the cabin with electricity, so this guide covers vehicles with a gas or diesel engine.

How your heater gets its heat

The heater borrows its heat from the engine. Hot coolant flows from the engine through the heater core, a small radiator tucked behind the dashboard, and the blower fan pushes cabin air across it. When you turn the temperature control, a flap called a blend door changes how much of that air passes through the heater core and how much goes around it. Some vehicles use a valve in the heater hose instead, or as well, to control how much hot coolant reaches the core.

That makes the heater part of the engine cooling system. Transport Canada's winter preparation advice puts a working heater and defroster on the same list as coolant strength and level, radiator hoses, drive belts, the radiator cap, the water pump and the thermostat, and CAA's winter car care list does much the same.

What the symptom points to

The last row is the one that can't wait. When a heater goes cold because coolant has leaked away, the engine can run short of coolant too. If the gauge climbs, a warning light comes on or you see steam, don't keep driving: pull over somewhere safe, turn the engine off and let it cool, and leave the radiator and reservoir caps closed while it's hot. An engine that has overheated needs checking before it's driven again, so call the shop at 604-793-9310 first.

The common causes

Low coolant

Low coolant is a common reason for weak heat: there isn't enough hot liquid reaching the heater core. The cooling system is sealed, so a low level means the coolant has gone somewhere: through a leaking hose, the radiator, the water pump or the heater core, for example. Mazda's owner's manual says that if the reservoir is empty or needs new coolant often, it should be looked at by a qualified repairer.

Strength matters as much as level. Coolant is a mix of antifreeze and water, and the mix decides how cold it can get before it freezes. Honda, for example, says its premixed 50/50 coolant protects to about -35°C, and its owner's manual tells owners not to add straight antifreeze or plain water. Other makers specify other coolants, so go by your own manual.

If you look at the level yourself, do it with the engine cold and read the marks on the see-through reservoir without opening it. Never open the radiator cap or the coolant reservoir cap while the engine is hot. Honda warns the coolant can spray out and seriously scald you, and Mazda says not to remove either cooling system cap when the engine and radiator are hot.

A thermostat stuck open

The thermostat is a valve that holds coolant back from the radiator until the engine is warm. If it sticks open, coolant goes to the radiator too early and the engine runs cooler than it should. The signs are a temperature gauge that sits low or takes a long time to climb, and heat that stays lukewarm and gets worse at highway speed in cold weather. On many newer vehicles the engine computer monitors warm-up, so a thermostat stuck open can also turn on the check engine light.

The AA notes a thermostat can also stick closed. That's the more urgent version, because coolant can't get to the radiator and the engine can overheat.

A blocked heater core, or air in the system

The heater core is a bundle of narrow tubes. It can clog if the cooling system isn't flushed or the coolant isn't changed regularly, and restricted flow means less heat or none at all. Your owner's manual sets how often the coolant should be changed.

Air trapped in the cooling system can keep hot coolant out of the heater core in much the same way, and it often follows a drop in the coolant level or work on the cooling system. A water pump that isn't moving enough coolant can starve the heater core as well.

Some advice online suggests flushing a heater core with a garden hose or bleeding the air out yourself. A flush doesn't show why the core blocked, and the job means opening a system full of poisonous fluid, so leave it to a technician who can find the cause first.

The blend door and controls

If the engine is warm and the coolant is fine, the heat may be there but not reaching you. The blend door is a flap inside the heater box behind the dashboard, and on many vehicles a small electric motor called an actuator moves it when you change the temperature. When the actuator or the door fails, the temperature may not change when you turn the control, one side of a dual-zone system may stay cold, or you may hear clicking behind the dash.

The blower and airflow

No air is a different problem from cold air. The AA points to the blower fan, a blown fuse or damaged wiring, and says a fan that works only on its highest setting usually means a fault in the fan-speed control. These are electrical faults, so the fix starts with electrical troubleshooting rather than a new part picked by guesswork.

Weak airflow with windows that fog easily can be as simple as a clogged cabin air filter. Honda's owner's manual says the filter may need replacing if airflow drops noticeably and the windows fog up easily. Mazda's operating tips add another: clear leaves, snow and ice from the hood and the air intake at the base of the windshield.

A sweet smell from the vents or a greasy film on the glass

A leaking heater core can let coolant into the cabin. Often the first sign is a sweet smell from the vents when the heat is on. It can also leave a slightly greasy fog on the inside of the windshield above the defrost vents. If you notice either, especially along with weaker heat or a coolant level that keeps dropping, have it looked at soon. The film makes the glass harder to clear, and a heater core that ruptures can spray coolant into the cabin. If coolant sprays into the cabin or steam comes from the vents, pull over safely and turn the engine off.

Weak heat means weak defrost

The defroster blows air from the same heater onto the glass, so a weak heater means slow defrost.

Defrost doesn't rely on heat alone. In the Mazda3 owner's manual, for example, selecting defrost switches to outside air and turns the air conditioning on automatically, because the A/C dries the air before it reaches the glass. The manual also warns that fog comes back more easily with the A/C off or in recirculate mode, and that defrosting with the temperature set to cold can fog the outside of the windshield. Its advice is to set the temperature to warm or hot.

So a windshield that won't defog can come from either side: a heater that isn't making enough heat, or an A/C system that isn't drying the air. On a wet Fraser Valley morning, damp air and wet coats can fog the glass even when it isn't especially cold.

When slow heat is normal

Slow heat on a cold morning is often the engine warming up. The heater can't be warmer than the coolant, and the coolant needs a few minutes of driving to warm up. A few things are normal:

• Automatic climate control holding the fan back. Mazda's manual says that when heat is selected, the system restricts airflow until the engine has warmed, so it doesn't blow cold air at you.
• On automatic climate control, maximum hot doesn't heat faster. The same page notes that setting the temperature to its hottest (or coldest) setting won't reach the temperature you want any sooner.
• Less airflow at a stop on stop-start vehicles. Mazda's operating tips say airflow may drop slightly while i-stop has the engine stopped.

Once the glass is clear, driving warms things up faster than sitting in the driveway: Natural Resources Canada calls a few minutes of driving the most efficient way to warm the engine and the cabin (NRCan fact sheet). How long to idle, and when a block heater helps, are covered in the guide to car batteries in cold weather.

A temperature gauge that never reaches its usual spot, heat that stays lukewarm on a long drive, or heat that used to come quickly and now doesn't all point back to the thermostat and coolant causes above.

Getting weak heat diagnosed

If the heater is blowing cold or the coolant level keeps dropping, book cooling system service and describe what you've noticed: when the heat drops off, what the temperature gauge does, and any smell, clicking or fogging. If you're getting the car ready for winter anyway, ask for the heater, defroster and coolant strength to be checked at your next maintenance appointment.

Common questions

Can I keep driving with no heat?

That depends on why the heat is gone, and only a check will show that. If coolant has leaked away, the engine may be short of it too and could overheat. If the gauge climbs, a warning light comes on or you see steam or smell coolant, stop somewhere safe and turn the engine off. In winter, no heat can also mean no way to keep the windshield clear. Have it checked before you rely on the car, and don't head out on a highway trip until you know the cause.

Can I just top up the coolant?

If the level is low, topping it up with the coolant your owner's manual specifies, with the engine cold, may bring some heat back for now. It won't fix the reason it was low, and heat coming back doesn't mean the car is safe to drive: if there's a leak, the level will keep dropping and the engine can run short. Have the cooling system checked for the cause.