Car heater not working: what causes it and what to check

If your car heater is not working, start with the coolant level and the temperature gauge. The heater uses hot coolant from the engine, so low coolant, or an engine that never warms up properly, is the most common reason for cold air from the vents.

Other causes are more specific: a blocked heater matrix, trapped air in the cooling system, a failed blower motor or a control flap that no longer moves. Some are simple, others need proper diagnosis, so it helps to notice exactly what your car is doing.

Why is my car heater blowing cold air?

Cold air with the fan working normally points towards the heat source. The most frequent causes are low coolant, air trapped in the system after a top-up or repair, a thermostat that is stuck open so the engine never reaches temperature, or a heater matrix that is blocked.

If the air is cold on one side of the car and warm on the other, that can suggest a temperature blend fault on cars with dual-zone controls.

• Low coolant level
• Trapped air in the cooling system
• Thermostat stuck open
• Blocked or restricted heater matrix
• Temperature or blend flap not moving

Why is my heater only lukewarm or slow to warm up?

A gauge that takes a long time to rise, or never reaches its usual position, often means the thermostat is stuck open. The engine runs too cool and has little heat to give the cabin. On some diesels, which make less waste heat, this is more noticeable in winter, but a car that used to warm up quickly and now does not has a fault worth checking.

A partly blocked heater matrix gives a similar result: the engine temperature looks normal, yet the air from the vents stays tepid, or the two heater hoses feel very different in temperature.

What can I check safely at home?

Only check the coolant when the engine is completely cold. Never open the cap on a hot engine, because the system is under pressure and can scald. Look at the level against the min and max marks on the expansion tank, and check your handbook for the correct type of coolant.

Then see whether the fan blows at every speed, whether the gauge reaches its normal position and whether there is damp carpet or a sweet smell inside the car. These observations are very useful to a technician. Please do not keep topping up a car that is losing coolant without finding out why.

Can a faulty blower or switch cause the problem?

Yes. If you get no airflow at some or all fan speeds, the blower motor, resistor or a fuse may be at fault. If the fan works but the temperature will not change, the blend flap, its actuator or a control fault could be responsible. These are different jobs from a cooling system repair, which is why a diagnosis comes first.

A blocked cabin filter can also reduce airflow and make a heater seem weak.

What does a garage do and when should you stop driving?

We would check coolant condition and level, look for leaks, test the thermostat operation, feel the heater pipes and test the blower and controls. Cost depends on the cause and how much of the dashboard or cooling system must be worked on, so we would ask you to describe the symptoms and quote after an inspection.

Stop driving if the temperature gauge rises into the red or a warning light comes on, if you see steam, or if the windows fill with a greasy film and a sweet smell, which can indicate a leaking heater matrix. Driving an overheating engine can cause serious damage.