Skip the auto shop - Our mechanics make house calls

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

P0121 OBD-II Trouble Code: TPS "A" Circuit Range Performance Problem
P0121 code definition Throttle Pedal Position Sensor/Switch (TPS) A Circuit Range Performance Problem What the P0121 code means P0121 is a generic OBD-II code which indicates that the Engine Control...
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 P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
Insurance Requirements for Car Registration in Kentucky
The Kentucky Transportation Cabinet requires that all drivers in Kentucky carry liability automotive insurance, or “financial responsibility” in order to operate a vehicle legally and maintain vehicle...


Related questions

Q: 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...

Q: Hold button flashing, no a/c,and temp gauge not functioning

Hello. This can be caused by a few different things. Most of the time it is caused by low refrigerant in the system, but I have seen a lot of these switches fail as well as the pressure switches in...

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...