Why Is Your Car Not Blowing Hot Air?

What’s more frustrating than a heater that won’t work in the middle of winter? I’ve been in situations where you crank the heater all the way up, and nothing happens. It makes your commute even more uncomfortable, and it could be dangerous depending on where you live.

There is something defective in your heater system. It could be the heater core, lack of coolant, blower motor, radiator, thermostat, blend door, blend door actuator, fuse, air filter, or the vent itself. Follow the rest of my guide to figure out the troubleshooting and solution to each of these individual problems.

In this guide, I want to help you get hot air back into your cabin. This expert guide is all about why your car isn’t blowing hot air from the vents. I’ll teach you how the heater works then go through 9 different reasons why you’re not getting hot air, how to troubleshoot it, and how to fix the problem.

How Your Car’s Heater Works

Even though people lump the heater and AC together as part of the whole HVAC system of your car, they use completely different processes (though there is certainly some overlap). In order to understand what’s wrong with your heater, I’ll start by teaching you how it works.

In reality, your car’s heater is actually tied in with the process that keeps your engine cool. It makes sense — using all the excess heat that gets pulled from the engine to heat incoming air kills two birds with one stone.

In this case, it’s coolant doing all of the work. Coolant has a very high boiling point and low freezing point. It stays as a liquid and absorbs a ton of energy from surrounding parts. It’s used to “suck” heat away from the engine, then keep moving. There are two different paths the coolant can take after it’s heated up from absorbing engine heat.

The first option is the radiator. Here, the coolant will lose a lot of its temperature and chill out. The other path is used as part of your car’s heater. It goes to the “heater core” through a control valve. The valve adjusts how much coolant goes to the heater core (if any goes at all). When your AC is on, this valve is completely closed.

The heater core works like another radiator in theory, but it works more like a space heater in this case. The coolant gets pumped through the core, and the top surface of the core gets really hot (since the coolant is so hot).

A blower motor will force air to pass across this hot surface of the core. The final destination? Your HVAC vents in your cabin.

This system works in junction with outside air that’s coming in. With “recirculation” off, you’re getting a healthy combination of outside air and air from the blower fan. The fan gets to blow softer since extra air is coming in. This helps the health of the blower motor and lets it last longer.

The Very Short and Simplified Version

If you’re easily confused, then here’s a quick and simple version that gets the point across; Your engine heats up a fluid that gets pumped into a separate part that heats up thanks to this hot fluid. Air is forced across the part and into your car, giving you hot air.

1. Problems with the Heater Core

The heater core is really the heart of your heating operation within your car. If the core is leaking fluid or having issues maintaining temperature, then the air will never heat up. It’s like trying to make eggs on a stovetop that isn’t turned on.

Some problems include holes in the system, mechanical damage, or a clog somewhere in the lines.

Troubleshooting

With a broken heater core, you’ll notice that your engine overheats, coolant gets drained quickly, cold air is flowing from your vents, and you smell something sweet in the air.

Solution

If it’s a clogged line, then you can flush the system and clear up the blockage. Otherwise, you’ll need to spend up to a thousand dollars or so to fully replace or repair the heater core.

2. Not Enough Coolant

As I mentioned, coolant is the magical fluid that makes all of this possible. It absorbs heat from the engine and transfers it over to the heater core, allowing your incoming air to heat up.

Without coolant, temperature and energy don’t get transferred. If there’s not enough coolant, then your air will get slightly warm but not nearly hot enough.

Troubleshooting

Start by checking the coolant reservoir towards the back corner of your engine bay. There will be a “fill” line. If the coolant level is below this line or not visible at all, you don’t have enough. You should also take a look under your car and check for signs of a leak.

Solution

The first key is to see if there’s a leak somewhere or if the coolant just naturally burnt off. If there’s a leak, you’ll need to replace the component or hose that’s leaky. Refill your reservoir up to the “fill” line, and ensure the new coolant has a 50/50 mix of coolant and anti-freeze.

3. Blower Motor Is Defective

The blower motor is responsible for blowing air over the heater core, so you have enough airflow. As you turn the fan speed up, you’ll get more air coming out of your vents at a higher speed.

This problem is more obvious when you’re in the recirculation mode. That closes the flap and doesn’t let air come in from the front of your car naturally.

Troubleshooting

You’ll know it’s the blower motor if you’re not getting any air coming out of your vents in recirculate. Turn your system to the defroster setting and check the airflow. If air still isn’t coming out, your blower motor is defective.

Solution

Replace the blower motor.

4. A Clogged Radiator

Even though the radiator isn’t a step in heating up your cabin, it’s part of the heater’s system. If the radiator is clogged, then the closed-loop system will have a bottleneck. This means that coolant won’t be moving around fast enough, stifling how fast hot coolant can be brought to your heater core.

Think of it like a closed exit on the highway. It might not be your exit, but it’s adding way more traffic to the highway and the next exit will be more congested.

Troubleshooting

Start by looking at your radiator. Often, the damage that blocks a radiator can be noticed from the outside. Fins might be damaged, or there might be excessive debris on the outside. Take a look at the coolant. If it’s thicker or darker than usual, there’s a problem with your radiator.

Solution

The easiest solution is to fully replace the radiator. Trying to repair it often costs more than buying a new one (depending on the damage).

5. Thermostat Is Stuck

The thermostat is a part that I haven’t mentioned yet. It’s the on/off switch that gets this whole process started.

It’s located in the coolant line, and it keeps everything stationary until the engine says that it’s getting too hot and needs to cool down.

If a thermostat doesn’t work, then the coolant won’t ever flow through your car’s system. It will sit stationary.

Troubleshooting

If your engine overheats while you’re driving or the coolant isn’t flowing after your car has been running, then your thermostat could be stuck. Alternatively, a stuck-open thermostat will force your engine’s temperature to always be too low, so the gauge will always be pointing towards the “C”, even after running for a while.

Solution

Follow my guide to unstick your car’s thermostat.

6. Faulty Blend Door or Actuator

The blend door is the place where incoming air mixes with forced air from the blower motor and heater core. This will also mix together different temperatures of air to ensure you get the right temperature, based on what you set on your dial in your car.

There’s an electrical actuator that controls the door opening and closing. These doors change as you move from defrosters to AC, heater, or heating your feet instead of your face.

The actuator waits for you to select the zone that you want air to flow through, then it will activate the correct blend door.

A blend door is partially mechanical and partially electrical, which makes it susceptible to failure.

Troubleshooting

First, check the airflow in different modes. If you get hot air at your feet but not your face, then it’s likely the blend door. Alternatively, you could be stuck in a single mode. If you change around the modes but the air still comes out of the defrosters, that’s another sign that the blend door is faulty.

Solution

The actuator should be tested and replaced if it’s faulty. The doors themselves should be checked for damage, especially along the pivot point.

7. Blown Fuse or Relay

I’ve mentioned a lot of electrical components thus far, so it makes sense that a relay or fuse could also be the root of your problem. Fuses act as sacrificial pieces for any electrical component in your home or car. Relays are essentially electrical switches that control when something turns on or off.

If there’s a surge of power that might fry a motor somewhere, your fuse will take the surge instead and burst before the electricity can go to the motor and do damage. Relays can stop operating once they wear out.

Fuses are only a few dollars, and potentially save you hundreds in repairs if the surge were to damage the motor down the line.

In the case of your heater, the blower is the one you want to check. 

Troubleshooting

If air doesn’t flow at all and you don’t hear grinding from the front of your car as you swap modes, it’s likely a blown fuse. You can check your fuses by locating your fuse box and looking through each fuse.

Solution

Follow my guide for replacing a worn or blown fuse.

8. Air Filter Needs Replacement

The air filter is part of your HVAC system, and it’s the final component before the air goes out of the vents and into your cabin. These air filters serve a single purpose: to purify the air before you breathe it in.

It takes out contaminants like pollutants, dust, pollen, and allergens.

Air filters work almost like a strainer. There are tiny holes in overlapping pieces of paper and metal. The holes are large enough for air to pass through but too small for airborne dust to pass through. It’s like a big wall that only lets through the air that’s safe to breathe.

As contaminants are strained out, they get trapped in the air filter. Over time, an air filter has so many trapped contaminants on it that there aren’t enough open holes for air to flow through.

With a constricted airflow, not enough hot air can pass through your vents.

Troubleshooting

Turn your heater fan all the way up and listen closely. If you can hear the motor blowing air, but you only feel a little bit of air, take a look at the setting. Ensure that recirculation is on and you’re on the “face” setting.

If you feel a slow flow of air from your face vents, then the filters could be too clogged.

Do the same but put turn your AC on instead of the heater. The air filter is shared between modes, so you should get the same slow flow of air.

Solution

Take out the air filter and replace it with a new one. This should be done roughly every synthetic oil change or two (every 15,000 to 30,000 miles).

9. The Vents are Clogged

Finally, let’s check out the vents in your cabin. These are the slatted plastic or metal pieces that you can adjust — where the air physically blows out.

If your vents are closed, then air is not going to flow. If they’re clogged with a lot of dirt and dust, then the airflow will be restricted.

The good news is that these vents are really easy to check out, you don’t even need to take anything off.

Troubleshooting

While sitting in your seat, crouch down and get eye-level with the vents. Use a flashlight and look for the reflective metal of the tubing on the other side of the vents. If you can’t see it, then there could be an obstruction or the vent could be closed.

Move the vents around and keep looking inside. Different cars have different ways to open and close the vents, so you might not even realize that the vent was closed.

Also, adjust the speed of the heater fan and listen closely. If you can hear a louder whooshing of air but still do not feel it from the vent, then the vent might be clogged.

Try all of the vents around the vehicle. It’s rare for more than one or two vents to be clogged at the same time. Your defroster and leg warming vents can’t be closed, so you should feel airflow from there as you swap to those modes.

Solution

First, try to move around the vent and open it. If it’s opened and you’re not getting airflow, use a small duster or pipe cleaner to get rid of the blockage. If that still isn’t working, then your vents are either mechanically damaged and blocked, or it’s not the vents that are broken.

Conclusion

At this point, you should know exactly why your car isn’t blowing hot air. I just explained how your heater works and outlined 9 different culprits, troubleshooting steps, and solutions to fix your problem.

If you have any other car questions, check out the rest of my site. I have plenty of troubleshooting guides and how-tos that might answer your question. Leave a comment below if this guide helped you at all. Also, be sure to see what car products I highly recommend, and pick some up for yourself.