Why is My Car Heater Not Working?

Can’t get heat from your vents? There are a number of reasons for a car heater not working properly. Although it’s a mostly annoying occurrence, it should be taken seriously because it can be a sign of other parts causing the issue. And if you can’t clear your windows, it’s actually a safety issue too.

Fixing the issue yourself won’t set you back more than a few hundred dollars at worst, but if you need to take your car to a mechanic, you can expect to pay for parts and labor up to $1,000, perhaps more, depending on the year, make, and model of your car.

The most common reasons for a car heater blowing cold air include:

  • A low coolant level
  • Plugged cabin air filter
  • Faulty heater fan
  • Faulty heater core
  • And a faulty thermostat

In this article, we’ll cover the reasons for the car heater blowing cold air, and what you can do to fix it.

Low Coolant Level 

One of the first things to check, and also one of the most common reasons why your heater isn’t working is a low coolant level. In optimal working conditions, the coolant level shouldn’t change but in the event of a leak somewhere, or blown headgasket, you could be consuming coolant. This, in turn could and will cause the heater core to not get the hot antifreeze it needs for heat-exchange, making your vehicle nice and warm. This can happen due to a cracked coolant tank, coolant hose, leaking water pump, or a head gasket. Other cooling system parts can also leak coolant, so it’s best to check them regularly. 

If your car doesn’t start blowing warm air in 10 to 15 minutes, the coolant level might be too low to circulate through the heater core. Make sure to check for leaks, because a low coolant level can also lead to the engine overheating, which is a much more expensive thing to fix. 

Coolant doesn’t evaporate, and just topping it off doesn’t fix the issue, you’re just buying yourself some time to dedicate more time to finding the real cause of the leak. A gallon of coolant costs around $20, so if you notice the coolant isn’t at an optimum level, make sure to top it off before driving the car. 

The most common causes for a low coolant level include: 

  • Cracked radiator or faulty radiator cap 
  • Faulty expansion tank 
  • Leaking water pump 
  • Blown head gasket 

Depending on the reason that caused the low coolant level in the first place and which parts need to be replaced in your car, fixing may cost between $20 and $800. 

Failed Heater Control Valve 

A Heater Control Valve is used to divert, or stop hot Antifreeze from being routed into the Heater Core, which produces heat for the vehicle. When you select AC or “Cool” temperature on your vehicle, most vehicles have a heater control valve which then stops or blocks antifreeze from flowing to the Heater Core. When a Heater Control Valve fails, one of two things happen – either it leaks, or it seizes in one position or the other. If it seizes in the closed position, hot Antifreeze will not make it to the Heater Core, and your heater will blow cold. Some vehicles use a manual valve controlled by a cable, while others are electric. Other vehicles don’t have one at all, and use different methods in the dash for shutting off heat to the Heater Core. An easy test here is to locate your Heater Control Valve, if your vehicle is equipped. With the vehicle running and up to temp, turn the heater on. You should feel BOTH heater hoses leading to the Heater Core as hot, or warm. If one of them is cold and not coming up to temp, its very likely that either the Control Valve is bad, or the Heater Core is plugged.  

Plugged Cabin Air Filter 

The cabin air filter is more important than most people think and serves the same purpose as your home furnace filter. The primary goal of this component is to prevent dust and other debris from reaching the cabin, however, it needs to be replaced regularly to prevent reduced airflow. Once the cabin air filter has restricted airflow due to trapping dirt and debris, it will also prevent warm air from reaching the cabin as well, and you’ll notice the car heater not working and less air will be coming through the vents. This is very easy to diagnose because you’ll notice a lack of airflow out of the vents no matter what selection you have – heater, AC, or with the temp set in the middle for “room temp” or vent.  

Fixing this issue is very simple and it doesn’t take more than a couple of minutes to replace the cabin air filter, no matter what year, make, or model your car is. If a plugged cabin air filter was the reason for the heater not working, replacing the filter with a new one should resolve the issue.  

The cost of replacing a cabin air filter can cost anywhere from $30 up to $60, however, you’ll save a lot if you do this by yourself. In most cars, reaching the cabin air filter isn’t difficult and it shouldn’t take more than 15 minutes.  

The Heater Fan or Fan Resistor  

Another common issue for blowing cold air is the heater fan. A heater fan on most vehicles is relatively easy to replace, but some fans are under-dash, inside the heater box. If you can’t find an issue with the wiring or the fuse, another culprit is a Blower Motor Resistor. This electrical resistor controls the various fan-speeds, but sometimes can completely fail. If you have a specific fan speed that is not working, then the resistor is almost always the culprit.  

Before ordering a new heater fan, you can test it as well using a voltmeter. First, check to see if 12 volts is getting to the positive connector on the fan. If it’s not, your problem is elsewhere. If it is, you can attempt to test the fan by connecting it directly to the battery of the vehicle using a test wire. If the fan won’t spin, it’s suspect.  

Faulty Climate Control Panel  

The vast majority of vehicles over the last 15 years have switched to all-electrical panels for Climate Control. Even if your vehicle still has traditional “dials” to control the temp and settings, the chances are this panel is 100% electrical inside rather than the cables of old. Your entire climate control panel can have an internal failure, causing the switches that control the blower and blend doors to work, to fail. While some of the very-common panels are available as complete new units at AutoZone, many are not and require either a replacement salvage unit, or a new one from the dealer. Before assuming the part is dealer-only, check at your local AutoZone.  

The Heater Core is Leaking or Blocked 

Your Heater Core is the source of heat inside your vehicle. It takes hot antifreeze and turns this into heat – no different than a small radiator. If your Heater Core has an issue, a number of different things can happen.  

First, a Heater Core can become blocked. Often, this happens because of old, neglected Antifreeze, and the fact that in most climates, the heater is only used for a few months out of the year. This causes sediment and corrosion to settle into the core’s passages, and create restrictions. Rememeber, Antifreeze has to properly flow through to allow fresh, hot Antifreeze into the core. A plugged heater core can often be remedied during a coolant flush, where the core is blasted out with fresh water. If this still doesn’t solve the problem, you’ll need to replace the Heater Core. One of the best maintenance items on your Heater Core is rather simple – every month in the summer, switch your vehicle to heat  

A faulty heater core can also leak, which causes a major problem. It can fog up the windows with Antifreeze mist, which is very dangerous. The vehicle will also have a sweet, Antifreeze smell in the cabin when the heater is running.  

Unfortunately, the Heater Core itself is one of the most labor-intensive repairs on your entire vehicle. It involves removing the entire dash, and on most modern cars, this is a massive task. Most Heater Cores are 10-15 hours of labor for an experienced mechanic, so tread carefully as to whether you want to take this task on or not.  

Broken Blend Door or Actuator 

This fault is becoming much more common on vehicles made in the last 20 years. On any vehicle, the heater and AC box will use a blend-door to switch between air flowing through the Heater Core or flowing through the AC’s evaporator.On cars of old, this was controlled by a manual cable. Today, these doors, and the doors to operate defrost vs vent vs floor are all operated by electric blend doors and servos, or motors, that open and shut these. Couple this with dual climate control, which also uses these doors to control the passenger getting a different level of heat or cool compared to the driver, and you have a recipe for a lot of different failure points. Some blend doors are easy to diagnose and change, while others are difficult. When the vehicle’s blend door is stuck on the AC side, the blower motor’s air flow will not circulate past the Heater Core, and thus, the vehicle will not generate any heat.  

Thermostat is Faulty 

Lastly, the thermostat can also be the culprit behind a car heater not working. The thermostat is inexpensive, but it has one of the most significant roles in a car. It helps keep the engine’s running temperature at optimum levels and directly affects the heating and cooling in a car. 

Replacing a faulty thermostat isn’t a very difficult job, especially if you’ve done some car maintenance before, but in some models, it can be tucked away in a hard-to-reach spot. Another thing to consider is that you’ll most likely need to replace the coolant when replacing the thermostat. 

Aside from the heater not working, you may also notice your car having trouble reaching its operating temperature. The unit itself costs between $20 and $80, but labor costs can reach be hundreds of dollars depending on your car. 

As stated, there are numerous things that can cause a vehicle’s heat to not work properly. While many of these are easy to diagnose, many are quite difficult as well. If you determine the job is too big, be sure to seek out one of our Preferred Shops in your area that can help you diagnose and do the job.  

Should you choose to do the job yourself, you can find all the parts you need to fix your heating and cooling systems at AutoZone!  

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Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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