How To Fix A Broken Car Heater

It's no fun getting into your car on a frosty winter's morning only to find the car heater not blowing hot air. It's almost as unpleasant to have a heater that's not giving out enough heat. Luckily, unlike many systems on the modern motor car, heating systems are more or less built around the same design as they were 20 years ago - which is ultimately a basic one. That said, there are multiple reasons why your heater could be busted - and working out which one it is can be tricky.

If the problem is simple - and you can diagnose it - then it's possible to fix it yourself. But some issues are more complex, meaning a visit to your local garage is necessary.

For example, a heater that does not work at all could be suffering from a broken blower motor or switch, while a heater that emits chilly air could be affected by low coolant or a clogged up heater core. With luck, any issues you have will become evident in warmer months, so you can fix them before winter sets in.

In most cases, heater issues fall under two headings:

1. Systems that blow air

There could be a blockage or other problem stopping heated antifreeze from moving through the heater core component.
A valve or switch could be faulty, or the heater core could be a clogged.
Wait until the engine has cooled down and check the coolant level, topping it up if needed.

2. Systems that simply don't work at all

In this case, you're looking at a faulty blower motor, or an issue stopping power from reaching the blower motor.

In order to fix this, you might have to prise open the heater box, or perhaps even lift a section of the dashboard to access the blower.

My heater has suddenly started to blow cold: what should I do?

Most car heater systems - i.e. ones that come with water-cooled engines - harness the coolant to heat the car interior. The coolant at this point is inappropriately named, since it is extremely hot. This fluid is pushed through the heater core, which is essentially a mini radiator. In turn, air is pushed through the heater core, which results in your cabin being filled with delightfully warm air - or at least, that's what should happen.

But if you feel the chill of cold air in your car, even after the engine has been running for some time (and the system is set to hot), then the following issues are likely:

Heater core is clogged

Coolant is low

Blend door is jammed/faulty

Heater valve is jammed/faulty

A switch or linkage is faulty

First of all, check the coolant level - but wait until the engine is completely cool, or you could be burned. If you discover coolant is low, there may an insufficient amount moving through your heater core.

While in the short term, topping up your coolant could address the issue, the fact it was low in the first place indicates another, more serious problem. It may mean a hose or gasket is leaking, or worse, that you have been burning the coolant itself, strongly suggesting a blown gasket.

If a refractometer indicates the pH is off, it's likely a component has suffered corrosion and begun to leak. Miscoloured coolant, or coolant that smells off, also suggests an issue.

In the case of having sufficient coolant, you can switch on your engine and let it heat up, before checking the temperature at the point where the heater core and hoses connect.

It is best to use an infra-red thermometer, since it does not require any contact. If you discover one of the hoses is the same temperature as the coolant but that the other hose is cold, it is likely your heater core has become clogged. If your car has a valve in the affected hose, you should check to see if it has got stuck.

What if hot antifreeze is in fact passing through my heater core?

In this instance, there could be dirt or other matter stuck in the heater box, or the blend door is jammed/faulty.

How to work out if my blend door is jammed or otherwise malfunctioning?

Turn off your heating and listen for the blend door moving. If you can't hear it move, you may be looking at an issue with the thermostat switch (if your car has one); a jammed hinge on your blend door; or a linkage/wiring fault.

Car heater not blowing hot air?

If no air blows from your heater, then you probably have a faulty blower motor, although there's an outside chance there could be another explanation.

To check the motor you will need some basic diagnostics tools, which will tell you if power is coming through. You will need direct access to the motor to do this. You might also be facing a faulty blower resistor; relay; or switch. Each car is different, so how you work out the problem will depend on your model. Check your owner's manual for more information.

If you believe power is reaching the motor, it's probable that it has burnt out. It's possible, too, that the motor has been clogged up with debris to the point that it cannot work. Rust and damaged wires can also be the culprit.

In the case that power is going to the motor, you should check the blower fuse, resistor, relay and switch. If you think it's the fuse, be sure to replace it with one of the same amperage. The fuse may have blown due to age, or it may suggest an underlying problem. If the replacement fuse (of the same amperage) does not blow, you may well have identified the problem. Do not be tempted to replace the fuse with one of a higher amperage to keep it from blowing again.

Or… let your garage fix it

For those without the necessary skills, mechanical aptitude or motivation, and for those who are simply too busy with other things, it may make a lot of sense to let your local garage fix the problem - particularly if the issue is difficult to diagnose.