Finding a Cheap Fix for a Broken Defroster

Frozen foggy windshield
Rolfo / Moment Open / Getty 

Automotive defrosting systems are absolutely essential in cold weather, but they're also indispensable whenever the combination of humidity and temperature conspires to fog up your windows. When your defroster stops working, reduced visibility can lead to some extremely dangerous driving conditions.

There are actually two different types of car defrosters, so tracking down, and fixing, this type of problem requires a different process depending on whether it's your front or rear defroster that stopped working.

What Causes a Car Defroster to Stop Working?

Since there are two types of car defrosters, the reason yours stopped working will depend on the type you're dealing with.

Front car defrosters typically use air from the vehicle's heating, ventilation and air conditioning (HVAC) system to melt ice and clear foggy windows, while rear defrosters typically rely on a grid of hot wires affixed to the window glass.

There are exceptions, but these types of defrosters are found in most cars.

Reasons for a Front Defroster to Stop Working:

  • Broken or stuck controls: The buttons or dial you use to switch between hot and cold, and switch which vents the air comes out of, can become stuck or break. Some of these even use gears or cables that can jam or get pinched.
  • Vent and air intake problems: If you can hear the blower motor running, but no air comes out of the defroster vents, the vents may be plugged, or the fresh air intake could be blocked.
  • Coolant problems: If your defroster only blows cold air, you could have low coolant in the engine. The thermostat could also be stuck, or the heater core could be plugged.
  • Blower motor problems: If you don't hear anything at all when you turn on your heater, air conditioning, or defroster, the blower motor could be bad. You could also have a bad switch or fuse.

Reasons for a Rear Defroster to Stop Working:

  • Broken defroster grid: Rear defrosters rely on a very thin grid of wires affixed to the window glass. If the wires are physically broken, the defroster won't work.
  • Grid worn out: If your car is old, the grid may just be too worn out to work properly.
  • Broken defroster connections: If the connections where power hooks up to the grid are broken, the defroster won't work.
  • Bad defroster switch or fuse: If the grid doesn't get power at all, suspect a bad switch or fuse.

Front Windshield Defroster Fixes

When you turn on your front windshield defroster, what really happens is that the HVAC blend door moves to direct air out of the dash vents. In some cases, turning the defroster on may also automatically activate your air conditioning.

When a front defroster stops working, it’s usually a faulty switch or blend door if air comes out of the other vents, or a bad blower motor if no air comes out of the vents. If air comes out of the vents, but it's cold even if you have the heat turned up and the air conditioning off, you have a problem with your cooling system.

The cost and complexity of those repairs depend on the vehicle, since some heater switches, blower motors, and blend doors are easy to get at, and others require you to remove the whole dash assembly.

Keep in mind that if your heat isn’t working, that doesn’t necessarily mean that your front defroster is also busted. Although blowing cold air from your A/C on your windshield isn’t going to melt any ice, it does effectively lower the relative humidity inside your car, which will do a fine job of defogging your windows on a cold, rainy day.

Rear Window Defroster Fixes

Unlike front windshield defrosters, rear window defrosters are actually dedicated devices that can, and do, break. They consist of relatively simple wire grids that receive power from the car’s electrical system when you flip the defroster switch.

When electricity flows through the grid, the wires heat up, which causes ice to melt and condensation or fog to dissipate.

The most common cause of a rear defroster failure is a break in continuity or a short in the defroster grid. The easiest way to check for this is to use a voltmeter or a test light to look for power and ground and to use an ohmmeter to check for continuity along each line of the grid.

Another common point of failure, especially in in hatchbacks, station wagons, and some SUVs, is the actual spade contacts where the power and ground are hooked up. Of course, it’s always possible for a switch to go bad as well.

When a rear window defroster goes bad, the repair is typically either expensive or time-consuming. Cheap repair kits can sometimes take care of continuity breaks, and aftermarket replacement grids are also available, but it is sometimes necessary to replace the back glass altogether.

If you see that a spade contact has become disconnected, but the spade is still affixed to the window glass, and the grid seems intact, try plugging it back in. There's a good chance your defroster will start working again.

Car Defroster Alternatives

In the case of front windshield defrosters, both heat and air conditioning can do the job of defogging your windows. So if one is working, and the other isn't, the best option is to just try to use the one that's actually functioning. If it works, then you may be able to put off an expensive repair.

Air conditioning gets the job of defogging done since the process of cooling air via an A/C unit pulls moisture out of it. Heat works because hot air is capable of holding more water than cold air, and cranking up the heat also heats up the glass of your windshield, which can prevent the moist air in your car from condensing there.

The efficiency of these two different methods depends on the local conditions, like how hot or cold it is outside and the relative humidity.

Electric car heaters can also do the trick, regardless of which type of windshield defroster you’re trying to replace. Although you’re unlikely to find a 12v or battery-operated heater that’s capable of replicating the heat output of your car’s heater core, some of these units are pretty good at defrosting and defogging windows.

If nothing else works, you can also check into 12v car defrosters.