Finding a Cheap Fix for a Broken Defroster

Don't get caught out this winter

Automotive defrosting systems are essential in cold weather, but they're also indispensable when the combination of humidity and temperature fogs up your windows. When your defroster stops working, reduced visibility can lead to hazardous driving conditions.

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

A frozen and foggy car windshield with a broken defroster.

Rolfo / Moment Open / Getty Images

What Causes a Car Defroster to Stop Working?

Since there are two types of car defrosters, the reason yours stopped working depends 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. In contrast, rear defrosters typically rely on a grid of hot wires affixed to the window glass. There are exceptions, but you can find these types of defrosters in most cars.

Here are reasons why a front defroster stops 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 use gears or cables that can jam or get pinched.
  • Vent and air intake problems: If you 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 the defroster only blows cold air, you could have low coolant in the engine, the thermostat could be stuck, or the heater core could be plugged.
  • Blower motor problems: If you don't hear anything when you turn on the heater, air conditioning, or defroster, the blower motor could be malfunctioning. It could also be a bad switch or fuse.

These are the reasons why a rear defroster stops working:

  • Broken defroster grid: Rear defrosters rely on a thin grid of wires affixed to the window glass. If the wires are physically broken, the defroster won't work.
  • Worn out grid: If your car is old, the grid may 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, the HVAC blend door moves to direct air out of the dash vents. Sometimes, turning on the defroster may also automatically activate the 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, there's a problem with the 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 the heat isn't working, that doesn't mean that the front defroster is also broken. Although blowing cold air from the A/C on the windshield will not melt any ice, it does lower the relative humidity inside the car, which will do a fine job of defogging the windows on a cold, rainy day.

Here's how to fix a front windshield defroster:

  1. With the engine off and cold, check the coolant level. If the coolant is low, fill it up. The defroster may start working again at that point, but there's an underlying coolant leak problem that needs to be fixed. If the front windshield is sticky and you can't wipe it clean, the heater core is probably leaking.

  2. Check the heating, ventilation, and air conditioning (HVAC) controls. If the push-button or dial controls don't move smoothly, it may be caused by bad controls, or something may be bound inside the dash. If you have vacuum-activated controls, there may be a break in the vacuum lines.

  3. Check if you can hear the blower motor running. If you can hear the blower motor but no air comes from the vents, check the fresh air intake. If it's plugged, clean it. If it isn't, the blend door may be stuck, or the vents may be plugged internally.

  4. Check for power at the blower motor. If you don't hear the blower motor running, then check for power. You may be able to fix it by replacing the fuse, but it's more likely a bad blower, a bad switch, or a bad ballast resistor.

Rear Window Defroster Fixes

Unlike front windshield defrosters, rear window defrosters are dedicated devices that can, and do, break. They consist of 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.

A 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 an ohmmeter to check for continuity along each grid line.

Another common point of failure, especially in hatchbacks, station wagons, and some SUVs, is the spade contacts where the power and ground are hooked up. 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.

Here's how to fix a rear car window defroster:

  1. Check the defroster grid. If you can see where the grid is broken or worn through, that's the reason the rear defroster isn't working. Some grids can be repaired, but you may need to replace the back glass.

  2. Check the spade connectors. Most defroster grids use spade connectors to provide power and ground, and they sometimes come unplugged. If the spade isn't broken off the window glass, try to reconnect it gently. If you can plug it back in, the defroster should start working.

  3. Check for power at the spade connectors. If there's no power or ground at the wires that connect to the spade connectors, it may be a wiring or switch problem. Trace the wires back to the source to determine if it's a broken wire or a bad switch, relay, or fuse.

Car Defroster Alternatives

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

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

The efficiency of these two 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.

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