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There’s almost nothing better than stepping into a warm, toasty car on a cold morning. It can be a real sanctuary from the brutal winter weather raging outside, so a malfunctioning heater can be a big bummer on a cold day. 

Your car’s heating system is tied closely with the operation of its cooling system, so a problem with one part could quickly evolve into a problem with the other. Understanding how heaters work is important, as is understanding what can cause them to fail. Car Bibles’ editors have spent time dealing with heating and air conditioning failures, so we know what to expect.

In this post, we’ll talk about how to recognize heater problems and what you can do to prevent them. We’ll also dig into the causes of heater problems and tell you how to track down heating issues in your vehicle. Winter’s coming, but we’re ready, so let’s get started.

Is the heat working?
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How Does My Vehicle’s Heating System Work?

Believe it or not, your heater isn’t all that complicated. No matter the vehicle, there are a few basic parts that every heating system needs to work. There’s a heater core, a fan, and a control system located in the cabin to allow the driver to change settings.

Your car’s engine generates heat as it runs. Most of that heat escapes through the exhaust pipe, but a portion of the heat is transferred to the coolant. As the engine reaches the proper operating temperature, the thermostat will allow coolant to circulate through the engine to “gather up” excess heat and move it to the radiator. The coolant also circulates to the heater core, which can distribute the warm air to the cabin.

Key Symptoms and Causes of A Broken Car Heater

  1. You notice a sweet smell inside your car. Radiator fluid can smell sweet, and when it leaks there can be a strong odor inside the car’s cabin.
  2. Your windows fog up for no reason. When a warm coolant leaks into a vehicle’s interior, it can evaporate and cover the windows with warm condensation.
  3. Cool air is coming out of vents. If the car’s engine is warm and the heat is turned on, warm air should be circulating. If it’s not, or if the air leaving the vents is cold, you’ve got a problem.
  4. You constantly need to refill coolant. If you’re seeing that the coolant levels are needing more attention than they used to, you might have a coolant leak.

You may also notice that the engine overheats more frequently than it did before and may see a check engine light in combination with the symptoms above. 

A radiator cap.
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Why Did My Heater Stop Working?

The most common cause of a failing heater core and heating system, in general, is dirty coolant. Over time, the coolant liquid can become contaminated with particles and dirt from its travels through the engine and cooling system. This can cause the heater core to clog and fail. 

That’s just one of the common reasons, so we dig deeper into additional causes of a failed heating system below.

Heater Core

In some cases, dirt and debris can build up and clog your car’s heater core. Particulates can get into the system with contaminated coolant or if there’s rust or damage to parts of the radiator, engine, or other mechanical parts.

Thermostat

Though often overlooked, your car’s thermostat is one of its most important components. It’s responsible for regulating coolant flow, so a thermostat malfunction can cause problems with everything from engine cooling to cabin heating issues. The thermostat’s ability to control the flow of coolant is important to heating systems, as a failure can cause a reduction in the heat transferred into the cabin.

Antifreeze or Coolant

If the level of antifreeze falls too far, the liquid can’t make its way to the heater core. This can happen for a variety of reasons, from leaks to improper fill levels. If the coolant can’t get to the heater core, there will be no hot air generated to heat the cabin.

Heater Fan

The heater fan is responsible for dispersing the air throughout the cabin. Even if every other part of your car’s heating system works properly, you won’t feel any warm air if the fan doesn’t work.

Radiator

A damaged radiator can leak and prevent coolant from reaching the heater core. Without the heater core, your car cannot heat up.

Climate Controls

The buttons and knobs that control your car’s heating and cooling systems can cause problems if they malfunction in some way. Climate settings depend on communication between the controls and the functional parts of the system, so if there’s a breakdown in that process, it can cause the heater to stop working.

Blower Motor Resistor

Your HVAC’s blower fan uses a resistor to determine the levels of air blowing into your cabin. If this fails, the blower motor will not work at all, and you’ll be stuck in an ice throne.

Wiring and Fuses

Almost every part of your car’s electrical system relies on a series of fuses and wiring. If a fuse blows, or if there’s damage to one of the wires, the heater might not work properly.

A thermostat.
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How To Fix A Broken Heater

Depending on the problem, the fix can be as simple as refilling the coolant or as complicated as a full-on heater core replacement. Let’s go over the most common fixes for automotive heating problems here.

Safety Notice: Working on your car, even on the little things, can get dangerous. To prevent damage to you or your vehicle, always use proper safety precautions. Always wear gloves and protective eye equipment, and always use jack stands when working underneath a lifted car. 

It’s absolutely possible to complete a full-on repair of your car’s heating system at home, but it’s important to remember that performing repairs like replacing the heating core is both involved and time-consuming. You’ll need to make sure that you have the right tools and a solid place to work before diving in to start any heater repairs at home.

Low Antifreeze

If your coolant/antifreeze levels are low, the easy fix is to check and refill it.

  1. Let the vehicle cool. Remove the cap and place a funnel into the overflow reservoir opening or the radiator opening.
  2. Add new coolant until the reservoir or radiator is filled. 
  3. Reinstall the cap. 
  4. Start the vehicle and turn on the heat. If the air blows warm when the engine is up to temperature, the coolant was the problem.

Note: Low coolant might be a symptom of a bigger problem. Be sure to monitor your levels going forward and inspect the areas where you park your car for signs of a leak.

Faulty Thermostat

If your car’s thermostat is to blame for its heating issues, you can do the job yourself in a few easy steps.

  1. Make sure the vehicle is off and cool. 
  2. Find the thermostat. It’s usually located near the base of the radiator by the main hose.
  3. Jack up the front of the vehicle and make sure it’s securely held by jack stands.
  4. Remove the radiator cap.
  5. Drain the radiator and coolant by removing the hose and letting the thermostat drip into a bucket.
  6. Remove the old thermostat and install the new one.
  7. Reattach the radiator hose.
  8. Refill the radiator with coolant.
  9. Squeeze the radiator hose to push out water bubbles.
  10. Reinstall the radiator cap.
  11. Lower the car and start the engine.
  12. Let the car come up to temperature and test the heater.
Damn, it's time to fix the heat!
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Car Bible’s Glossary for Heating, Ventilation, and Air Conditioning Terms

HVAC

This term refers to heating, ventilation, and air conditioning. In a vehicle, this means the climate control system. Though they’re referred to as a group, heating and air conditioning are generally two separate systems in a vehicle that don’t operate together.

Heater Core

A heater core looks a lot like a small radiator, but its function is opposite that of a radiator. Instead of dissipating heat, heater cores act almost as a concentrator of heat before the blower motor and fan transfer the warm air into the vehicle’s cabin.

Air Conditioning

Air conditioning uses a condenser and other components to cool air and push it into the vehicle’s cabin.

Radiator

Radiators are devices in vehicles that take in hot coolant liquid and use a series of fins and grates to dissipate heat out into the atmosphere. The coolant then circulates back into the engine, where it gathers more heat for the radiator to cool.

Refrigerant

Refrigerant is a liquid used to “charge” an air conditioning system. The names and formulations have changed over the years, but the basic idea is still the same: through evaporation, refrigerants can super cool the air around them.

Fan

Fans are used in vehicles’ climate control systems to move air from one place to another. In the case of this discussion, fans are used to push warm or cool air from the climate system into the vehicle’s cabin.

Thermostat

Thermostats are used to monitor temperatures, both in climate control systems and in engine control systems. 

Your Questions, Our Answers on Car Heaters

Talk to the experts.

Q: What’s The Difference Between Heating And Air Conditioning Systems?

A: Besides how the air actually feels, there are a number of differences in how the heating and air conditioning systems work. The air conditioning system uses a compressor and refrigerant/coolant to generate and move cool air into the vehicle’s cabin. 

In cooling systems, a refrigerant is pushed from the compressor to a condenser, which removes heat and changes the refrigerant from a gas or vapor to a liquid. From there, the refrigerant is moved to an evaporator, where a blower motor blows air across. The air is cooled by boiling and evaporating refrigerant, and can be blown into the cabin to cool it down.

Q: I Don’t Mind Not Having Heat. Can I Just Ignore This Problem?

A: You might not need heat, but your vehicle’s cooling system needs to get rid of it. Driving around with a bad heater core or failing heating system can cause overheating and other issues that will cost more money and can’t be ignored.

Q: How Much Will A Fix Cost Me?

A: This will really depend on what, exactly, is wrong with your heating system. In general, count on spending between $300 and $1,000 on repairs related to the heating system, but the totals can end up being more or less if it’s a particularly big or tiny issue.

Q: Is Heater Repair Something I Can Do Myself?

A: Of course you can, but you’ll want to have at least a rudimentary idea of what you’re doing. Even if you’re an experienced home mechanic, it’s best to refer back to your vehicle’s maintenance manual to ensure you’re following the proper steps and buying the right parts for your vehicle.

Q: How Can I Prevent Heater Issues?

A: The biggest thing you can do to prolong the lifespan of your vehicle’s heating system is to make sure the coolant is clean and flushed regularly. It’s also important not to ignore warning signs of a failing heating system, as waiting can allow other parts to fail and cause damage.

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