Why Is My Car Heater Blowing Cold Air?

Charlotte Phillips, 6 months ago

3 min read

  • Diagnostic
Car heater

WhoCanFixMyCar explores the reasons why your car heater may blow cold air.

Car heaters stave off the bitter chill of winter, keeping us toasty even when temperatures plummet. They're also handy for defrosting, speeding up the process of getting out on the road. 

When they stop working, arranging a repair is often a top priority. This guide explains why your heater might be blowing cold air and what you can do about it.

Contents

How does your car heater work?

Why is my car heater not working?

What are the signs of a broken car heater core?

How much does it cost to repair your car heater?


How does a car heater work?

The heating in your car works in a fairly simple way.

  • Your vehicle’s engine begins to heat the coolant to a high temperature.

  • The coolant then moves through the heater core, which acts like a small radiator.

  • Air exits the heater core having been warmed up.

Why is my car heater not working?

There are a four main reasons why the heater may have stopped blowing hot air in your car.

1. Low antifreeze level

If your antifreeze level is too low, your heater will blow cold air into your car because there isn't enough hot liquid in the heater core. Check the reservoir - we explain how in this guide - and top up as needed.

Engine coolant

2. Heater core problem

The heater core is a cooling system that often looks like a compact radiator. It is made of metal tubing that transports coolant and several fans to disperse heat. Sometimes, the heater core becomes clogged, resulting in:

A car heater core.

It may be possible to fix this problem by cleaning and unclogging the heater core; otherwise, it will have to be replaced.

3. Dirty air filter

A dirty air filter will impact the heating system's efficiency because it won’t allow air to flow through the heater core, meaning it blows out of the vent cold.

Air filters need replacing every 12,000 to 15,000 miles.

iStock-air filter

4. A fault with the thermostat

The thermostat controls a valve that alters the coolant flow to maintain the correct engine temperature. It can go wrong due to ageing and corrosion, getting stuck in a closed or open position.

Close up of a car thermostat

When the thermostat fails to signal that the engine is warm, coolant won't be sent to the heater core, making the air blow cold.

5. Damaged wiring

A blown fuse or short-circuited wire could be to blame for your heater's lack of responsiveness. This problem is reasonably easy and affordable to fix.

An auto electrician carrying out wiring repair

6. Defective heater fan

Even if hot liquid reaches the heater core, you won't feel the benefits when there isn't a functioning fan to blow warm air into the car. An electrical or mechanical issue could cause a malfunctioning fan - it's hard to tell until you have it inspected by a mechanic.

What are the signs of a broken car heater core?

There are a few tell-tale signs that your car’s heater core is going bad.

Sweet smell in the cabin 

A sweet smell lingering in the cabin could indicate that the heater core is leaking and blowing the scent of engine coolant through your vents. Coolant is said to have a sweet and musty smell.

Recurring coolant loss

If your coolant seems to be constantly running low after topping it up, there’s likely to be a coolant leak in the heater core, which can make your engine overheat. It’s important to get this checked out by a professional as soon as possible.

Cold air blowing from vent

Cool air from your vents when the heater is switched on clearly indicates that something is going wrong.

Fog inside the cabin

When your heater core stops functioning correctly, demisting the windscreen becomes much more challenging, resulting in a fog building up inside the cabin.

How much does it cost to repair your car heater?

Luckily, if your car heater stops working, it’s a relatively inexpensive component to fix. While prices vary, the average quote for a car heater repair booked through WhoCanFixMyCar is £78.42.

If you need your car heating repaired, many garages in our network can do this. Enter your registration and postcode on the homepage to get a quote from a local specialist.

If you have any other issues with your car that you'd like to try to diagnose yourself, check out our guide on self-diagnosing car problems. The more you know about how your car works, the better.


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