Maintenance Tips, Tricks And Hacks For Your Vehicle

5 Reasons Why Your Windshield Won’t Defog




It’s a frustrating feeling when you can’t see out of your windshield. You turn on the defroster and wait, but nothing seems to happen. Your car starts to fog up again! What’s going on? In this blog post, we will discuss 5 common reasons why your windshield won’t defog and how to fix them.

Your windshield won’t defog if you have a corroded heat exchanger that is leaking, the AC is not working, a broken blend door actuator that controls the airflow, wrong settings, or the engine may not be hot enough to produce hot air.

Key Takeaway

  • A corroded heat exchanger that isn’t allowing heat to enter the cabin.
  • The AC is not working.
  • Broken blend door actuator that isn’t able to redirect the airflow to the windshield.
  • Wrong settings. Set the fan speed to max, redirect the flow to the windshield, and adjust the temperature depending on what is causing the fog.
  • The engine may not be hot enough. If the engine is cold, there will be no hot air inside the cabin.

5 Reasons Why Your Windshield Won’t Defog

5 Reasons Why Your Windshield Won't Defog

Driving with a foggy windshield can be both dangerous and frustrating. Even though you are trying, the windshield just won’t defog. Here are five reasons why your windshield won’t defog.

1. Corroded heat exchanger

5 Reasons Why Your Windshield Won't Defog

A heat exchanger plays an important role in any car’s cooling system. It regulates the temperature of the engine’s fluids, including coolant and oil. The device works by transferring heat away from heated areas, such as around the engine block or exhaust manifold.

Incoming coolant is directed through a series of channels and passages. There it absorbs heat from the hot components before being returned to the main recovery tank.

At the same time, the air is sucked inside by a fan attached to the exchanger. As this air passes through, it also absorbs heat which is then released as exhaust outside of the vehicle. The cooled air is finally pushed into the vehicle’s cabin.

There it keeps a comfortable temperature for both driver and passengers. With its blend of improved efficiency and safety features, no wonder why the heat exchanger has become an essential component in today’s cars.

Every driver knows the importance of being able to see clearly from the windshield and being free from fog or steam. Unfortunately, a faulty heat exchanger can make that difficult.

A bad heat exchanger’s primary purpose is to move excess condensation away and out of the vehicle. When it fails, it can no longer draw in air and warm it up before passing it through the cabin vents.

This means that warm, moist air isn’t being filtered away by the heat exchange process. This leads to a foggy windshield even when the temperature is at its hottest. To effectively clear your car of fog, you’ll need to diagnose and replace any broken parts of the AC system.

To test for a corroded heat exchanger, you need to wipe the windshield with a clean cloth and then smell it. If it smells sweet, it means that the heat exchanger is leaking coolant and is actually bringing that coolant inside the cabin.

This is very dangerous because it creates a layer on the windshield that prevents the driver from seeing. Also, it is bad for the lungs.

2. The AC is not working

5 Reasons Why Your Windshield Won't Defog

A car’s windshield can quickly fog up due to the ambient temperature and moisture levels. This makes visibility difficult and potentially dangerous.

Fortunately, cars are now equipped with a simple yet highly effective tool to combat this problem: the air conditioner (AC).

The AC works by using a fan to draw in and circulate air around the car while also controlling its temperature. If the AC is set to “recirculate,” it will use a sensor that detects interior moisture levels and adjusts the humidity accordingly.

Lowering the humidity inside the car, also lowers external fogging on the windshield. Additionally, if your AC has air-directional vents, you can target them at your windshield for better defogging results.

If you’re driving on a foggy morning and your windshield won’t seem to clear, it could be the fault of your AC. The AC not only keeps your car comfortably cool on hot days. It also helps clear up the fogged-over windshield.

This is because the AC draws in outside air and then passes it over cold evaporator coils. Since this process removes any humidity from the air before it’s blown out of the vents, when you direct airflow while defogging towards the windshield, the condensation quickly evaporates.

Unfortunately, if your AC isn’t working properly due to a lack of coolant or worn-out parts, it won’t be able to reduce the humidity level inside your vehicle effectively. The result is foggy glass that no amount of circulating air can remediate.

As such, if you find yourself stuck with a fogged windshield during humid days — or at any other time — consider having your AC serviced as soon as possible to ensure its proper operation and help eliminate foggy glass for good.

3. Broken blend door actuator

5 Reasons Why Your Windshield Won't Defog

There’s nothing worse than getting into your car and seeing that your windshield has fogged up. You might think that rolling down the windows or turning on your defroster should help to clear up the problem.

But, if you’re hearing a clicking noise coming from behind the dashboard, that could mean the blend door actuator needs to be replaced.

When you adjust the temperature in the cabin, either higher or lower, the blend door actuator will physically move within your car’s ventilation and heating systems in order to direct warm or cool air accordingly. Without a blend door actuator, you wouldn’t be able to adjust the interior temperature.

The blend door actuator is responsible for controlling the airflow inside a vehicle and ensuring it flows into the vents at the right temperature. If it has become stuck in one place due to wear and tear, air won’t be able to flow freely and cause a defogging issue despite all of your efforts.

Replacing this part is essential to keeping your car functioning properly. This is especially important during harsh weather conditions, as it can drastically reduce visibility if left untreated.

Keep an eye out for any signs of malfunctioning actuators. This way you can make sure that what appears on your windshield stays outside!

4. Wrong settings

When you have cloudy breath on a cold morning, you know it’s time to defog the windshield. A defogger windshield can make all the difference in avoiding danger on the roads when visibility is low.

However, if your car’s climate control is incorrectly set, your windshield won’t be able to effectively defog itself.

The air conditioning should be set to maximum recirculation and the temperature should be completely at one extreme or the other—either hot or cold—depending on what’s causing the fog. On top of these settings, you’ll likely want to press the defogger button too. This will channel warm air onto an internal grid that sits behind your windshield and disperse it evenly through small holes in its patterns.

Make sure the fan speed is up high enough especially during winter months when there’s more condensation in the air. Those tiny holes are then able to do their job right!

Once you’ve figured out how to adjust your climate control properly, you can get back on driving.

5. The engine is not hot enough

As drivers know, it is necessary to keep the windshield clear of fog during wet or cold weather. This can be a challenge if the engine is not hot enough. It won’t be able to generate enough warm air to defog the window.

Unfortunately, when this happens, the only available solution is to wait for the engine temperature to raise enough. Only then it will have an effect on the windshield temperature.

Until that happens, many drivers need to resort to other tactics such as cracking open a window and wiping away the fog with a cloth. Additionally, some vehicles have defogging settings that may help if enabled.

This setting uses special sensors in the car that detect moisture in the air outside and recirculate this air throughout the cabin in order to make it drier and reduce fogging.

Of course, these auto settings don’t work if they are not enabled or need repair, so another option would be to use a dehumidifier device placed inside the car.



Vide Polowenski, Senior Mechanic

The information in this article is current and up-to-date in accordance with the latest mechanic SOPs.

Please take the time to leave a comment if this article has helped you in any way, you need additional help, or you have a suggestion.

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