Skip the auto shop - Our mechanics make house calls
  1. Home
  2. Articles
  3. How Do Secondary Defoggers Work?

How Do Secondary Defoggers Work?

secondary defogger

Your car is equipped with two defoggers/defrosters. The primary one is in the front, and it’s tied into your air conditioner. Turn the controls to defrost, set your temperature, and then turn on the fan and you can watch as the moisture evaporates.

There’s a second defogger in the back of the vehicle on your rear glass (note: not all vehicles have secondary defoggers). However, it doesn’t work in the same way. Rather than forcing air over the glass, you flip a switch and then watch as lines form in the condensation, before it eventually clears away completely.

How secondary defoggers work

Actually, they work on the same principle as a lightbulb, and many other electronic components in your car – resistance. The secondary defogger is actually an electronic circuit. The lines you see in the glass are really wires, and they connect to the vehicle’s wiring harness.

When you flip the switch or push the button in the front that activates the defogger, power is sent through the system. The wires in the glass resist the small amount of current, which heats them up. They don’t heat up to the point that they glow, like a light bulb filament, but the principle is the same. Contact a mechanic if the defogger switch isn't connecting.

The heat from this resistance helps to equalize the temperature difference causing the fogging, clearing it away and giving you a clear view out the rear glass. Of course, like any other electronic system in your car, your secondary defogger is subject to wear and tear. A single damaged wire that leads to the defogger can render it inoperable.

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

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...
How to Transfer a Car Title in Idaho
In order to prove ownership of a car, you must have the title. However, when a car is sold, given away or inherited, the title needs...
P2422 OBD-II Trouble Code: Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed
Diagnostic Trouble Code (DTC): P2422 P2422 code definition Evaporative Emissions Control System (EVAP)...

Related questions

Q: How do you work the Rear Window Defroster?

Your car is equipped with both front and rear defrosters. While the front defroster is controlled with the fan speed/blower knob and ties into your air conditioner, the rear defroster is different. Here’s how to work it: Locate the...

Q: Q: AC blowing warm air

Hi. The problem your described is an indicator that the coolant fan for the condenser is not working properly. If the fans are working properly, I suggest having the refrigerant pressure check by an EPA certified technician. If you need...

Q: Are Power Car Windows Safer?

Power windows are a convenience feature, though there are elements of passenger safety that are incorporated. Power windows can be controlled by the driver, so if a passenger opens the car window when they shouldn’t, the driver can close it....