Car Heater Not Working or Blowing Cold Air: Causes and Fixes

Your car heater does not make heat on its own. It borrows it from the engine: hot coolant flows through a small radiator behind the dashboard (the heater core) and a fan blows air across it. If the air coming out of the vents is cold, one link in that chain is broken. Most of the time the cause is related to the cooling system, so it is worth treating the problem as more than a comfort issue.

What the symptom tells you

• Cold air and the temperature gauge stays low: the engine is not reaching normal temperature, which often points to a thermostat stuck open.
• Cold air and the temperature gauge is normal or high: coolant is not reaching the heater core. Low coolant, trapped air or a clogged heater core are the usual suspects.
• Heat on one side only, or only some of the time: low coolant, air in the system or a failing blend door actuator.
• Warm air, but weak airflow: a blower motor, resistor or cabin air filter problem rather than a heat problem.

The most common causes

• Low coolant. The most frequent reason. A leak somewhere in the system lets the level drop until the heater core no longer gets enough hot liquid.
• Air trapped in the cooling system. It often appears after a coolant change or repair that was not bled properly and creates cold spots in the heater core.
• Thermostat stuck open. Coolant keeps circulating through the radiator, so the engine runs cool and the cabin never gets warm, especially on cold days or at highway speed.
• Clogged heater core. Rust and old coolant build up inside the narrow passages. One hose to the heater core will be hot and the other much cooler.
• Blend door or actuator. This small door mixes hot and cold air. If it sticks you may hear clicking from behind the dash and get the wrong temperature.
• Weak water pump. A worn pump does not push enough coolant through the system, and the engine may overheat as well.

What you can check yourself

1. Look at the temperature gauge. If it stays near cold after 10 to 15 minutes of driving, suspect the thermostat.
2. With the engine completely cold, check the coolant level in the overflow reservoir. Never open the cap on a hot engine, the system is under pressure and can cause burns.
3. Look for puddles under the car, a sweet smell or a wet passenger footwell. Those signs point to a coolant leak, possibly in the heater core.
4. Foggy windows with a greasy film on the glass also suggest coolant vapor from a leaking heater core.
5. Test whether the fan speeds change. If airflow is weak or missing, the problem is the blower, not the heat.

When it is more than a comfort problem

If the heater stops working and the temperature gauge climbs, pull over and shut the engine off. Driving an overheating engine can warp the cylinder head or blow the head gasket. A defroster that no longer works is also a safety issue in winter because you cannot clear the windshield.

What a shop will do

• Check the coolant level, condition and look for leaks with a pressure test.
• Test the thermostat and replace it if it does not open and close correctly.
• Bleed the air from the system, or do a coolant flush if the fluid is old or contaminated.
• Flush or replace a clogged heater core, and check the water pump if flow is weak.
• Diagnose the blend door actuator and its wiring.

Start before the cold weather arrives. Compare nearby shops, read recent reviews from other drivers and ask the shop to explain which of the causes above they found.