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Your car can have a few systems to defrost its rear window, outside mirrors, and windshield. While they’re all important on frosty mornings, a windshield defroster is one of the most vital. So, how does it works, what are some maintenance tips, and what do you do when your car defroster stops working?

How does my car defroster work?

A window defroster at work on a ice covered window.
Window defrosted | Getty Images

Your car’s windshield defroster is part of the vehicle’s climate control system. As your car’s engine warms up, it circulates coolant, which is a blend of distilled water and antifreeze, through the engine’s coolant system. The coolant system uses the water pump to circulate the hot coolant through a series of rubber hoses and valves to the radiator and heater core when needed.

Depending on your car’s design, setting the climate control dials to deliver heat either opens a valve to send hot coolant to the heater core or your car’s heater core may get hot coolant continuously. Either way, the heater core heats up, and the climate control system’s fan blows air through it to deliver hot air through a series of ducts to the windshield. 

While warm air blowing on the inside of your windshield helps to thaw any accumulated frost on the outside, it does little to dissipate interior condensation that “fogs up” your windows. RepairPal says automatic climate control systems use the car’s air conditioner to reduce humidity inside, allowing the defroster to use warm, dry air to melt the frost away.

If your vehicle has a manual system, turn on the heat and air conditioning simultaneously and set it to pull air from the outside instead of recirculating. In addition, lowering your windows a bit allows moisture to escape with either system. 

Why did my car defroster stop working?

Since your car’s defroster employs multiple systems, mechanical parts, and sensitive electronics, a failure of any component could cause it to stop working. Here are five common causes, what to do when they happen, and some strategies to keep your defroster working.

1. Coolant level

Low engine coolant level is the most common and easiest-to-fix problem that plagues your car’s defroster. However, only check your coolant level when your engine is cold after it’s sat unused for several hours, as hot coolant can spray out and cause severe burns otherwise. 

You should keep watch for any puddles under your car and investigate to determine their cause. It’s also critical to check your engine coolant level frequently and refill it with the appropriate fluid. If you’re unsure what to do, automotive parts stores, dealership service centers, and local mechanic shops are good places to ask for instructions. 

2. Thermostat

Your car’s thermostat stays closed until the engine heats up to its operating temperature. When the engine is warm, it opens to allow coolant to flow through the system and closes again if the engine gets too cool. If it gets stuck open, the coolant won’t get warm enough to supply warm air to the defroster, and if stuck closed, it won’t allow coolant to circulate. Warning signs include the heater blowing cold or lukewarm air, engine overheating, and abnormally high or low engine temperature if your car has a temperature gauge. 

Radiator cap

As the engine coolant heats, it causes increased pressure in the system. The radiator cap controls the pressure by venting if it becomes too high, but sometimes radiator caps wear out and vent prematurely. A faulty radiator cap causes low coolant levels, and the reduced system pressure lowers your heater’s efficiency. 

Don’t ignore a leaking radiator cap! If your engine temperature appears within its limits, you can buy a new radiator cap to see if that fixes it. Alternatively, you could have a repair shop pressure test your coolant system. 

Heater core blockage

The inside of a heater core consists of tiny passageways that allow hot coolant to circulate and dissipate heat. As engine coolant ages, it breaks down, becomes contaminated, and turns into sludge while losing its effectiveness as a coolant, antifreeze, and corrosion preventative. Old coolant leaves deposits that clog the small passages in the heater core, preventing heating, and its reduced corrosion prevention rots the heater core from the inside out. 

The good news is that flushing your engine’s coolant system, as indicated in your owner’s manual, is a moderate DIY project. You can complete the process in an afternoon in your driveway with a few standard hand tools. Drain the old coolant into a pan and pour it into empty antifreeze jugs or any container with a lid when finished to transport it for recycling, or have a shop complete the entire process. 

Fan not working

If the fan doesn’t come on when it should, the most common cause is a blown fuse. Use your owner’s manual to find the location of the suspected fuse and replace it if blown. Take caution if the new fuse fails immediately or within minutes, as this could indicate a more serious electrical problem with severe consequences.


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