How Do Secondary Defoggers Work?

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.


Next Step

Schedule Rear Window Defogger Switch Replacement

The most popular service booked by readers of this article is Rear Window Defogger Switch Replacement. YourMechanic’s technicians bring the dealership to you by performing this job at your home or office 7-days a week between 7AM-9PM. We currently cover over 2,000 cities and have 100k+ 5-star reviews... LEARN MORE

SEE PRICING & SCHEDULING

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

Recent Rear Window Defogger Switch Replacement reviews

Excellent Rating

(149,537)

Rating Summary
140,027
5,194
1,055
749
2,512
140,027
5,194
1,055
749
2,512

Christopher

20 years of experience
247 reviews
Christopher
20 years of experience
Subaru Forester - Coolant is leaking - Zephyrhills, Florida
Honest & professional
BMW 328i xDrive - Spark Plugs - Land O' Lakes, Florida
Punctual, friendly, and knowledgeable.

Nick

66 years of experience
96 reviews
Nick
66 years of experience
GMC Yukon XL 1500 - Window Motor / Regulator Assembly Replacement (Driver Side Rear) - Sacramento, California
He did a good job

Damian

11 years of experience
465 reviews
Damian
11 years of experience
Honda CR-V - Tie Rod End Replacement (Front Left Outer, Front Right Outer) - Odenton, Maryland
Good mechanic, explained services performed. His performance was a good representation for my first experience with this online booking service.......I saved $200 versus a big franchise service company.

Chris

16 years of experience
296 reviews
Chris
16 years of experience
Subaru Outback - Pre-purchase Car Inspection - San Diego, California
Great!!!! He was very informative and meticulous!!!! I think I’m going to request him as our mechanic all the time!!!

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

Related articles

Does a Cracked Window Need to Be Replaced?
Windows Windows are not just for looking through. Though it may seem that visibility is the sole purpose of your car’s windows, they are also for structural support. Every piece of glass on your car -- the windshield, rear window,...
How Much Does the AC Affect Fuel Mileage
Your car’s air conditioner is an important accessory – it helps keep you and your passengers comfortable and...
How Does a Defroster Work?
We’ve We’ve all been there. You slide behind the wheel, crank the engine and then stop. You realize that you really can’t go anywhere because your windshield is fogged up. Thankfully, you can just turn on the defroster and let...

Related questions

Car overheating and not defrosting properly.

Hi, I have seen this issue quite often on the PT Cruiser. This is typically caused by a failure in the heater core that is located in the dash. This part carries hot coolant into the interior of the car...

AC not blowing cold

You likely have two issues occurring at the same time. The fan operation is most likely due to an issue with your blower fan speed controller. It could also be the control assembly with the knob, or plug and wiring....

How do I use the auto up/down windows?

If your vehicle is equipped with the auto up/down windows, then you can roll the window all the way down or all the way up with only a quick press of a button. Press the Power Window switch down to...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1 (855) 347-2779 · hi@yourmechanic.com