Car Heater Not Working? Common Causes and Fixes

Summary

• Low coolant is the most common cause: The heater uses hot coolant to warm the cabin. If the level is low, there's not enough hot fluid to circulate.
• A faulty thermostat costs £100–£250 to replace: If the engine takes a long time to warm up, the thermostat is probably stuck open.
• Heater core problems are the most expensive: Replacing a heater core can cost £300–£800 because of the labour involved. See our servicing guide for maintenance tips.

It's January. It's freezing. You get in the car and blast the heater — but nothing happens. No warm air. Just cold misery.

Here's what might be wrong and what you can do about it.

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Common Causes of Heater Problems

1. Low Coolant Level

The heater works by passing hot engine coolant through a small radiator (heater core) behind the dashboard. If the coolant level is low:

• Not enough hot fluid reaches the heater core
• You get lukewarm or cold air
• Fix: Top up the coolant. If it keeps dropping, you have a leak — get it checked.
• Cost: Coolant top-up: £10–£20 DIY. Leak repair: £100–£400.

2. Stuck Thermostat

The thermostat controls how hot the engine runs. If it's stuck open:

• The engine never gets hot enough
• The coolant stays too cool to warm the cabin
• The temperature gauge stays low
• Fix: Replace the thermostat.
• Cost: £100–£250 including labour.

3. Blocked Heater Core

Over time, the heater core can get clogged with debris and old coolant:

• Reduced or no hot air from the vents
• Sometimes a sweet smell in the cabin (coolant leaking inside)
• Fix: Flush or replace the heater core.
• Cost: Flush: £50–£100. Replacement: £300–£800 (labour-intensive).

4. Blend Door / Flap Motor Failure

A motor-driven flap directs air through the heater core. If the motor fails:

• Air comes out but it's always cold (or always hot)
• You might hear a clicking sound behind the dashboard
• Fix: Replace the blend door actuator.
• Cost: £100–£350.

5. Blower Motor Failure

The blower motor pushes air through the vents. If it fails:

• No air comes out at all — not hot, not cold
• Or it only works on certain speed settings
• Fix: Replace the blower motor or resistor.
• Cost: Resistor: £50–£150. Motor: £100–£300.

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Quick Checks You Can Do Yourself

1. Check coolant level: Look at the expansion tank when cold. Is it between min and max?
2. Check the temperature gauge: Does the engine reach normal operating temperature? If not, suspect the thermostat.
3. Check all fan speeds: Does the fan work on every setting? If only some work, the resistor may be faulty.
4. Feel the heater hoses: With the engine warm, both hoses going into the bulkhead should be hot. If one is cold, the core may be blocked.

Never drive without enough coolant. Low coolant can cause engine overheating, which is far more expensive than a heater repair.

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When to See a Mechanic

See a mechanic if:

• You're losing coolant and can't find the leak
• The engine temperature stays low or overheats
• You smell coolant inside the car
• The windscreen keeps misting up (possible heater core leak)

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