Windshield Keeps Fogging Up? Sweet Smell from Your Car Vents? Tips to Diagnose & Fix the Heater Core

If your windshield keeps fogging up or you have a sweet smell from your car vents, there might be a problem with the heater core. Follow these tips to find out why this happens and how you can fix and prevent a foggy windshield or sweet smell in your car.

Diagnose & Fix: Windshield Keeps Fogging Up and/or Sweet Smell from the Car Vents

Mechanic in a car with a windshield that keeps fogging

How a Heater Core Can Cause a Windshield to Keep Fogging or a Sweet Smell in Your Car

Mechanic holding a heater core
Heater core

It’s common for a bad heater core to cause problems like the windshield fogging up. The heater core connects to two hoses through the firewall so hot coolant or antifreeze can pass through it. When the heat is turned on, the blower motor pushes air through the heater core, and this hot air then enters the cabin.

Heater Core Is Leaking

One sign of a heater core leak is finding coolant leaking beneath your vehicle from the drain pipe. The drain pipe connects to the heater box, and coolant will leak from it if there’s one at the heater core. If the heater core is leaking, the lack of coolant will result in a lower temperature, which means colder air will blow into the cabin.

Heater Core Is Clogged

If the heater core is clogged, the restricted flow of coolant or lack of circulation will lead to colder coolant reaching the heater core, and then colder air drifting into the cabin.

More on diagnosing a heater core

How to Fix a Clogged or Leaking Heater Core

Heater box with blower motor (left), AC evaporator (center), and heater core (right)
Heater box with blower motor (left), AC evaporator (center), and heater core (right)

The heater core is in the heater box along with the AC evaporator, blower motor, and door actuators. It can be time consuming to replace it, and you’ll need to remove parts like the dash.

Flush a Clogged Heater Core

If the heater core if clogged, you can try flushing it with tubes and a garden hose. You’ll need to disconnect the hoses from the heater core and attach tubes in their place. Then flush the water through the heater core in the opposite direction it normally flows, and let the water flow out the other end.

More on flushing the heater core

Replace a Leaking or Damaged Heater Core

Follow the step-by-step instructions in our how-to videos and learn how to replace parts like a leaking or damaged heater core yourself and more.

More Tips

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Windshield Keeps Fogging Up? Sweet Smell in Your Car? - 1A Auto
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Windshield Keeps Fogging Up? Sweet Smell in Your Car? - 1A Auto
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If your car has a windshield that keeps fogging up or a sweet smell inside, find out how a bad heater core can cause this and what to do with these expert tips from professional mechanics
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1A Auto
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