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Why does my windscreen fog up?

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Updated 10 Oct 2019

Jason Unrau

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Early morning or late at night, when the temperature is cool and the humidity is high, it’s common to get a bit of fog on the outside of your windscreen.

But what about fog on the inside of your windows?

It’s usually weather-related as well and should clear up quite quickly.

But if you’re having trouble keeping your windscreen clear on the inside, you might have problems.

What causes your windscreen to fog up on the inside?

Here are a few problems that can happen and tips to address it.

Improper Heater Settings

When the air outside your car is cooler than the air inside your car, condensation can form on the inside surface of the glass.

It’s basic science, isn’t it? Usually it takes just turning up the fan and adjusting the heat to clear the fog.

If you’re still having trouble clearing the fog, you might have the heater settings improperly adjusted for the condition.

To best clear your windscreen, turn the mode dial to defog – the icon with the wavy lines.

It directs air from vents on top of your dash up against the glass to dry the moisture.

Air Conditioning Problem

Your foggy windscreen could be a problem with your air conditioning too.

In cool weather, when your heater is set to defog, the air conditioning system kicks in.

Air conditioning does more than just cool your car’s interior – it also removes moisture from the cabin.

If the A/C system doesn’t cycle normally, the fog on your windscreen isn’t taken away, rather just circulated inside your car.

A/C problems that can contribute to foggy windows include a failed A/C compressor, a leaking evaporator, plugged receiver/drier, or an air conditioning system that’s low on refrigerant charge.

It could also be a blown fuse.

In some vehicles, one or more heater control buttons will flash when the A/C system isn’t working.

Leaking Heater Core

That fog on your windscreen may not be fog at all. It could be engine coolant.

Your car’s interior heat comes from the heater core, a radiator-style heat exchanger that has hot engine coolant flowing through it.

It’s possible for the heater core to spring a leak, either from corrosion or a flaw in the system.

A fine mist from the heater core enters the cabin from your vents and the coolant can settle on your windscreen.

If the fog is due to coolant on the glass, it doesn’t disappear nearly as quickly.

Plus, you’ll notice a distinct sick-sweet smell inside the car.

You may also notice a puddle of coolant on the passenger side floor, or the low coolant light can come on.

Water Inside the Car

Did you just have your car’s carpets steam-cleaned?

The extra water inside the cabin will eventually turn to vapour and need to go somewhere.

Until it does, it’s going to fog up your windows.

How Do You Deal with a Foggy Windscreen?

First, check that your heater controls are set to defog, not floor or vents.

And if you’ve just had your car detailed, crack the windows to let the moisture out.

Otherwise, have a trained mechanic on AutoGuru diagnose the condition.

It could be more serious than it looks at first.

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Written By

Jason Unrau

Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.

One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.

Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.