Skip the auto shop - Our mechanics make house calls

How the Defroster Works

Car Defroster

Whether it’s on a cold winter day, or it’s humid outside and the front or rear windows are fogged, having a reliable working car defroster is vital to maintain visibility. A fully functional vehicle defroster is a valuable component for your car, especially on those cold winter days when you have frost or ice on your windshield. While older models only have defrosters on the front windshield, many newer models have them on the rear window as well to improve visibility for drivers.

The actual components used to activate the front and rear window defrosters differ depending on what year, make, and model of vehicle you own. In general, the information below will provide an outline as to how these systems work.

What is the Job of a Window Defroster?

There are two different types of window defrosters: front windshield defrosters and rear window defrosters. The front windshield defroster is designed to blow large volume of air on the windshield with the goal of dissipating condensation that has built up on the inside of a windshield. When the weather is cool, droplets of water can form on a vehicle’s windows. Condensation on the inside of the windshield occurs because the air outside is colder than the temperature inside the car. When the temperatures become even colder, the condensation turns to frost or ice, which must be scraped away by hand or thawed by the defroster.

How Front and Rear Window Defrosters Work

In simple terms, the front defroster works by circulation of air, while the rear defroster is electrically charged. The front defroster has vents on the dashboard that face the windshield and front windows. The fan and blower motor that operate the heating and air conditioning will also circulate air through these vents to defrost the windows.

The process of operating the front defroster is unique to your vehicle. In general, to activate the front defroster, all you have to do is make sure the vents are open, turn on the fan and turn the setting to defrost and set the temperature to the desired setting. In most cases, warmer air blowing on the inside of the window will expedite this, but on the first engine start of the day, it will take time to create heat.

The rear defroster is electric for most vehicles. The rear window will have thin lines running across the window. These lines are electrical fibers embedded in the glass that heat up as they are activated. This defroster has its own button that you access when you want to defrost the rear window. You will notice the condensation or ice dissipating first along the lines until the entire window is clear.

How the Defrosters are Activated

Front defrosters work best when the air blowing onto the window is warm. However, it takes time for heat to build up in the engine and activate the heater core. When the coolant reaches a certain temperature, it will open the thermostat. Hot water will move through the core of the heater while the blower fan pushes the warm air through the vents of the defroster to warm up the windows. The condensation or ice will begin to dissipate as the window reaches the desired temperature. If the heater is not working, the front defroster will have difficulty working correctly.

The rear window defroster is electrically powered. The lines on the rear window are electrical. They heat up when the rear defroster is activated and begin removing the condensation immediately. The bonus for an electric defroster is that it begins working as soon as you turn the vehicle on and push the rear defroster button. Many new models include electric defrosters along the edges of the front windshield to enhance the defrost system and remove condensation faster.

Heated side view exterior mirrors also use electrical defrosters to remove condensation so that you can view the area surrounding your car. The difference is that you don’t see visible lines as with the rear window defroster. Note that these heaters provide a low amount of heat and won’t burn you if you touch the window while they are activated.

Common Issues with Defrosters

You often won’t notice a problem with a defroster until you need it and it fails to work. This can happen for a variety of reasons:

  • Buttons or knobs that are stuck or have stopped working – may need replaced or repaired
  • Blown fuse – when a circuit is overloaded, it may blow the fuse that connects to the defroster, the fuse can be tested and replaced by a professional
  • Missing terminal edges on the window – this may be caused by tinted windows that have started cracking or tint that has come off
  • Lack of antifreeze – when the antifreeze level is too low, the car may not heat properly or allow the defroster to work
  • Frayed wires – wires that are disconnected or frayed will prevent the defroster from working
  • Clogged vent – when a vent is clogged with dust and debris, the air cannot flow through to warm up the windshield

If your front or rear window defroster is not working, it is a good idea to contact a professional mobile mechanic to come to your location and complete a car defroster is not working inspection. This will allow them to pinpoint what is broken or not working so the right repairs can be completed quickly.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

How Long Does a Vacuum Brake Booster Check Valve Last?
The braking system on your car requires a lot of pressure. The vacuum booster is one of the main sources of this pressure. This booster will take the pressure that...
How Long Does a Throttle Return Spring Last?
Not all vehicles on the road feature an electronic throttle control (ETC), which is also called drive-by-wire. For vehicles that don't offer this system, they have a throttle cable instead that...
Symptoms of a Bad or Failing Flexible Clutch Hose
Common signs include difficulty shifting, low clutch fluid, and feeling no resistance at the clutch pedal.


Related questions

Q: How do you defog the side windows?

While your windshield and rear window both have dedicated defogging systems, the same can’t be said for your side windows. However, there are ways to combat moisture buildup on the inside of your windows, as well as frost on the...

Q: Door ajar light on and doors only manually lock

All of the lock functions and rear wiper functions are controlled by the Body ECU. If the ECU is failing this could be the source of the issues. In the front door and in the rear door, there are both...

Q: How do you use the defroster?

It’s important that you have a clear view out of the windshield and all the other windows in your car. Of course, it seems to take very little to fog them up, and during the winter, ice and freezing rain...