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What to Do When Your Car's Defroster Stops Working

Stephen Fogel
November 19, 2018

defroster not working

You know you’ll need to wait a minute for your defroster to warm up when you start the car on a cold day. But what happens when you keep waiting — and waiting — only for it to never get working?

Your defroster is important. If you can’t see clearly through the windshield as you drive, that’s a big safety issue. But if it’s not warming up, it can be a symptom of a much larger problem. Components about to fail, drivability problems or even a blown engine could be on the horizon.

Let’s go over how the defroster is supposed to work, or you can jump ahead to learn about why it fails and what to do about it.

How your defrosting system works

The defroster is a part of both the cooling system and the air conditioning system. Both play a part in getting moisture off the inside and outside of your windows. 

The cooling system circulates coolant through the engine, absorbs heat, and circulates the heated coolant through the radiator and the heater core in the dashboard. This keeps the passengers warm, and it can also be directed onto the windshield to defrost it.

The air conditioning system helps dry the air inside. In addition to keeping you cool when it’s warm out, the air conditioning also removes moisture from the interior surfaces — handy for defrosting. 

Hot, dry air does the best job of defrosting — the heater gets it hot, and the air conditioner makes it dry. For the best results with a manual system, turn the heat up all the way, turn on the A/C, set the blower fan to maximum, and open your windows slightly. If you have an automatic climate control system, simply set it to maximum defrost, and crack the windows.

Make sure that your air recirculation switch is off when you need to defrost your vehicle. Leaving it on traps all of the accumulated moisture inside the car, making it difficult to clear the glass.

If your defroster isn’t working

A problem in your defrosting system can usually be traced to one of the following: 

  • The cooling system
  • The heater core
  • The heater valves or blend air doors
  • The air conditioning system
  • The blower fan  

» LEARN MORE: Find a certified mechanic in your area

The cooling system

Coolant level

If your coolant level is low, it could keep your heater core from getting enough coolant to produce heat.

Solution: When the engine and radiator are cool, top up the coolant. Then warm up your vehicle and see if the defroster starts working better. 

If the coolant level is low due to a leak, have a mechanic find it and fix it. Without enough coolant, your engine will eventually overheat, leading to a much more expensive repair.

Thermostat

The thermostat is a valve in the cooling system that is supposed to stay closed until your engine reaches its operating temperature. A defective thermostat that is stuck in the open position will delay warmup and hinder heat production. 

Solution: Have a mechanic test the thermostat to see if it’s working properly. If it’s defective, it should be replaced.

Leaking coolant hoses, loose clamps

Over time, coolant hoses deteriorate. The clamps that secure these hoses can also loosen. A visual inspection of all the coolant hoses and connections will tell you if everything is secure. Also look for hoses that feel “spongy” if you have an older vehicle — this can mean a failure is imminent. 

Solution: Replace all of your coolant hoses, make sure all clamps are tight, and check for leaks.

Radiator leak

This may be obvious when the car is idling or parked, so look for puddles of coolant under the front of your vehicle. You may also find a dripping or wet area on the radiator.  

Solution: The radiator might be fixable; otherwise, a replacement is necessary.   

Radiator cap

The radiator cap regulates the pressure in the cooling system, acting as an escape valve if the pressure gets too high. It’ll wear out eventually, and if it sticks in the open position, there will be insufficient pressure in the system, reducing the heater’s output.

Solution: A leaking radiator cap can indicate more serious problems, so have a mechanic check this right away. It could be as simple as replacing the cap.

Water pump

The water pump moves the coolant throughout the engine, radiator and heater core. On older vehicles, it can be a source of leaks or other problems.

Solution: You may need a new water pump if it’s not working right.  

Engine fan

Most cars today have an electric fan that comes on the engine gets too hot. A defective switch could let the fan run continuously, reducing the coolant temperature to the point where you can’t get enough heat into the interior. 

Solution: A mechanic can replace the switch or sensor that controls the fan.

Air lock

This isn’t a part — it’s a large air bubble that forms in your cooling system as the result of a coolant leak or a recent top-up. An air lock prevents the coolant from circulating properly and can cut your heat output.

Solution: Air locks can be tricky to fix correctly, and can do serious damage if left untreated. Let a mechanic deal with this situation.  

The heater core

Heater core internal passages 

The heater core is like a miniature radiator built into the dashboard. Air passing around the heater core is warmed by the hot engine coolant. But the narrow passages can become clogged. This is usually because the coolant has not been replaced in a long time. 

Solution: A mechanic can flush out the heater core’s passages with either a water hose or compressed air. If this does not resolve the problem, a replacement heater core may be necessary.

Heater core exterior

The heat-radiating fins on the outside of your heater core could also be clogged with debris. This can affect the output of the heater and defroster. A clogged cabin air filter may also be the culprit. 

Solution: A mechanic can clean the debris from the fins and the air intake passages, and replace the cabin air filter if necessary.

The heater valves and blend air doors

Heater valves

These are the controls that regulate the output of the heater core. They can be mechanical or vacuum-operated (like a rotary knob that you turn) or electronic (as with electronic climate control systems with specific temperature settings). A valve that is stuck in the closed position will prevent heat from reaching the defroster.

Solution: The mechanical or vacuum-operated variety can usually be repaired by your mechanic, or you may need a new valve or valves.

The electronic systems are more complicated, as they’re usually integrated with the air conditioning system. These systems often have blend doors, which allow various combinations of heat and A/C. A malfunctioning blend door could close off the flow of heated or cooled air to the defroster. A mechanic can examine the system and identify the problem. 

» LEARN MORE: Get an estimate for your car repair

The air conditioning system

Refrigerant leaks

If your refrigerant level is low due to a leak, it could keep your air conditioning system from producing enough dry air to defrost your vehicle. 

Solution: Once the source of the leak is found and repaired, refrigerant can be added to the system to get it back in order.

Bad compressor

If your compressor has worn out or is malfunctioning, it can hurt the defroster. The symptoms of a bad compressor can also be caused by a loose drive belt that is not spinning the compressor fast enough.

Solution: The mechanic can check and replace the compressor if it’s not working right. If the symptoms are caused by a loose drive belt, the belt can be tightened or replaced.

Damaged, blocked or clogged condenser

The condenser is the equivalent of the radiator in your cooling system. Like a radiator, it removes heat from the refrigerant, cooling it before it returns to the system. If it is damaged, clogged inside, or blocked on the outside with debris, it won’t sufficiently cool the refrigerant.

Solution: A mechanic can inspect the condenser, clear out blockages and remove any external buildup. Or, if needed, you can get the condenser replaced.

Electrical problems

The air conditioner has a complex wiring system. Fuses and relays can also be involved. If one of these components fails, the system can stop working.

Solution: A repair shop will check the wiring and related parts to determine the cause of the problem, and then fix it.

The blower fan

Blower fan

If the car’s blower fan isn’t working, you won’t get enough heated or cooled air onto the windshield to defrost it.

Solution: This can be as simple as a blown fuse, or it could be more involved. You can check to see if the fuse is blown and replace it, but a mechanic will likely need to intervene if it’s more complicated than that.

Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.

1 User Comment

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By , September 14, 2018
I am interested in getting a repair to fix the blower accuator. The blower is working fine in all positions except for the fresh air vents on the dash.