A car heater that blows cold air is usually caused by one of five things: low coolant, air trapped in the cooling system, a thermostat stuck open, a blocked or leaking heater matrix, or a fault with the blower fan or the heater controls. The symptoms point to which one. Most of these show up in October and November, when drivers in Burgess Hill and across Mid Sussex turn the heater up for the first time in months.
This guide explains how the heater works, what each fault looks like, what you can safely check yourself, and when the problem needs a garage.
How does a car heater work?
A car heater uses heat from the engine. Coolant warmed by the engine is pumped through a small radiator behind the dashboard, called the heater matrix, and the blower fan pushes air across it and out of the vents. If hot coolant does not reach the matrix, or the air does not pass across it, the cabin stays cold.
That means a cold heater is rarely a fault in the heater alone. It is usually a sign of something in the engine’s cooling system, the fan, or the controls that decide where the air goes. Working out which of the three is at fault is the quickest route to the cause.
Is the coolant level low?
Low coolant is the most common reason a car heater blows cold air. The heater matrix sits high up in the cooling system, so when the level drops it is often the first part to run short of hot coolant. The cabin goes cold before the engine shows any sign of trouble.
Coolant does not get used up in normal driving. If the level keeps dropping, it is leaking from somewhere: a hose, the radiator, the water pump, the heater matrix itself, or, less often, inside the engine. Signs to look out for:
- A low coolant warning light on the dashboard.
- Puddles or drips under the front of the car, often green, pink, blue or orange.
- A sweet smell around the engine bay after a drive.
- A temperature gauge that climbs higher than usual.
Topping up will often bring the heat back, but only for a while if there is a leak. Our guide to the benefits of a car coolant flush explains what coolant does and why old or diluted coolant causes problems of its own.
Could there be air in the cooling system?
Air trapped in the cooling system can stop coolant flowing through the heater matrix, even when the level looks correct. An air pocket tends to collect at the highest point of the system, which on many cars is the heater matrix.
This often follows a coolant top-up, a coolant change or a repair on the cooling system. Typical signs are:
- A gurgling or sloshing sound from behind the dashboard, especially when you pull away.
- Heat that comes and goes, warm one minute and cold the next.
- A heater that only works when the engine is revved.
Some cars purge small amounts of air on their own after a few drives. Others need the system bled with the right procedure for that engine, and an air lock that keeps coming back can point to a leak or, in some cases, a head gasket problem.
Is the thermostat stuck open?
A thermostat stuck open lets coolant flow through the radiator before the engine has warmed up, so the engine never reaches its normal running temperature. The heater then only ever gets lukewarm coolant, and the air from the vents stays cool.
The thermostat is a valve that stays closed while the engine is cold and opens once the engine is warm. When it sticks open, you will usually notice:
- The engine takes much longer than usual to warm up.
- The temperature gauge sits lower than normal, or never reaches its usual position.
- The heater is warm around town but fades to cool on faster roads, where cold air through the radiator cools the engine further.
It is easy to ignore because the car still drives normally. It is worth having checked, though, because an engine that runs too cold uses more fuel and does not warm the cabin or clear the windscreen properly.
Is the heater matrix blocked or leaking?
A blocked heater matrix restricts the flow of hot coolant, so the air comes out lukewarm even when the engine is fully warm and the coolant level is correct. A leaking matrix is a different fault with its own giveaway signs inside the car.
A blocked matrix builds up over time when coolant is old or has been mixed with the wrong type, leaving sludge and scale in the narrow passages. The heater gets gradually weaker rather than failing overnight.
A leaking matrix lets coolant escape into the cabin. Signs include:
- A sweet, syrupy smell inside the car, often strongest with the heater on.
- A damp or wet carpet in the passenger footwell.
- A greasy film on the inside of the windscreen that smears when you wipe it.
- Coolant level dropping with no puddles under the car.
One side hot and one side cold can come from a partly blocked matrix, but on cars with dual-zone climate control it is just as often a fault with the flaps that mix hot and cold air for each side. The symptom alone does not tell you which.
A leaking matrix should not be left. Coolant vapour inside the cabin causes stubborn misting, and our guide to why your windscreen keeps misting up covers how a sweet smell and greasy film fit into that.
Is the fan only working on some speeds?
A heater fan that only works on its highest setting almost always points to a failed blower motor resistor. The resistor controls the lower fan speeds, while the top speed bypasses it, so the fan keeps working on full after the resistor has failed. On cars with automatic climate control, a blower control module does the same job and can fail in a similar way.
If the fan does not work on any setting, the cause is more likely to be:
- A blown fuse or a faulty relay.
- A worn blower motor, which may squeal, rattle or work only after a tap on the dashboard.
- A wiring or connector fault.
A fan fault feels different from a coolant fault. With a fan problem, you get little or no air from the vents. With a coolant problem, the air flows normally but comes out cold.
Could it be the heater controls or blend flap?
When the engine is warm, the coolant is full and the fan blows strongly but the air will not get hot, the fault may be in the controls. A flap behind the dashboard, often called the blend flap, mixes hot air from the matrix with cold air, and if it sticks on the cold side the cabin stays cold.
On most modern cars the flap is moved by a small electric motor called an actuator. Signs of an actuator or flap fault include:
- Turning the temperature dial or button makes no difference.
- A clicking or tapping noise from behind the dashboard, especially when the car is started or the setting is changed.
- Air coming from the wrong vents, or refusing to switch between screen, face and footwell.
These faults are often found with a diagnostic check, since climate control systems store fault codes much like the engine does.
What can you safely check yourself?
You can safely check the coolant level, the fan speeds and the footwells yourself. Anything beyond that, including bleeding the system or removing parts, is best left to a garage.
- Check the coolant level with the engine cold. Never open the coolant cap on a warm engine. The system is under pressure and hot coolant can spray out and cause serious burns. With the engine cold, the level in the expansion tank should sit between the minimum and maximum marks.
- Top up with the right coolant. Use the type given in the owner’s handbook. Mixing types can cause sludge, which leads to the blocked matrix problem described above.
- Run the fan on every speed. Note which speeds work and which do not.
- Check the relevant fuse. The handbook shows which fuse covers the heater fan.
- Feel the carpets. A damp passenger footwell with a sweet smell points to a leaking heater matrix.
- Watch the temperature gauge. Note where it settles on a normal journey and whether it ever climbs higher than usual.
Writing down what you found makes the problem quicker to trace when the car comes in. For the rest of your cold-weather checks, our winter car care checklist for Burgess Hill drivers covers coolant and antifreeze, tyres, the battery, wipers and screenwash.
When should you have the heater looked at?
Have the car checked straight away if the heater problem comes with a rising temperature gauge, coolant loss or a sweet smell inside the car. These point to a cooling system fault that can damage the engine if it is ignored. A heater that has simply grown weaker over the autumn is worth checking before the coldest weeks arrive.
A cold heater is not just uncomfortable. It also affects how well the windscreen demists, and the Highway Code asks drivers to make sure windows are demisted thoroughly before setting off. Stop and get help if:
- The temperature gauge rises towards hot or a warning light comes on.
- You can see steam from under the bonnet.
- The coolant level keeps dropping after topping up.
Frequently asked questions
Is it safe to drive with the car heater blowing cold air?
It depends on the cause. If the temperature gauge reads normally and no coolant is being lost, the car is usually safe for short journeys, though the windscreen will be slower to clear. If the heater went cold along with a rising temperature gauge, coolant loss or a sweet smell, stop driving and have the cooling system checked, because the engine could overheat.
Why is my car heater only warm when I am driving?
A heater that is warm when driving but cold at idle usually points to low coolant, air in the cooling system or a weak water pump. At higher engine speeds the pump pushes more coolant through the heater matrix, which can hide the problem. At idle, the flow drops and the air goes cold. Checking the coolant level with the engine cold is the first step.
Can low coolant stop the car heater working?
Yes. Low coolant is the most common cause of a car heater blowing cold air. The heater matrix sits high in the cooling system, so it runs short of hot coolant before the engine does. If the level keeps dropping, there is a leak that needs finding, since topping up only fixes the symptom.
Need your heater checked?
If your heater is blowing cold, keeps losing heat or comes with a sweet smell or damp carpets, we can check the heater and cooling system and track down the cause. Wheelbase Garage looks after drivers in Burgess Hill, Haywards Heath and across Mid Sussex. Get in touch to book a heater check.