Car Heat Only Works While Driving – What To Do

For most people, it seems like a season can't go by without having to deal with some kind of a mechanical issue. One of the worst examples of this is when your car's heater isn't working properly in the winter. A common and often perplexing version of this is when your heater only works while you're driving your car. We have learned all about car heaters and this specific issue so that you can stay warm in your car whether it's moving or not.

Several different problems can cause this issue to occur. This list will help you troubleshoot the issue and learn how to possibly fix it: 

  1. Low coolant
  2. Malfunctioning thermostat
  3. Air in the system
  4. Faulty heater valve
  5. Radiator leak 
  6. Blocked heater core

This issue is uncomfortable, inconvenient, and possibly very dangerous depending on just how cold out it is. To locate what is causing this to happen, it's important to have a basic understanding of how your car's heating system works. You can work your way down the list but it may not do you much good without knowing every aspect of each situation. Keep reading below to get an in-depth look at each possible problem and learn how to fix them!

A car heater display turned on the car dashboard, Car Heat Only Works While Driving - What To Do

Identifying And Fixing The Problem

Like any mechanical issue, the culprit can come from a variety of places. That's why it's important to narrow it down to a specific list so you can start working through it. This is the best way to identify what is causing your heater to malfunction. As with any mechanic work, make sure you have the proper tools and equipment to remain safe. Let's take a close look at each of the possible causes for this issue and how to possibly fix them. 

Three air conditioning blower inside a car

1. Low Coolant

At first, you may not think that coolant can affect your car's heat. However, having issues with your coolant is one of the most common causes of heater malfunctions. Your car's heater works by basically taking some of the hot coolant and putting it in the heating core. A fan then blows the hot air created by that heating core into your car. So, having low coolant means there may not be enough of it to divert into the heating core which can cause issues like this one.

To figure out if this is the cause of your headache you can do a couple of things inside, and outside of your car. While in your car if you hear abnormal sloshing or notice that your engine is running hotter than normal, then there is probably a coolant issue. You can also simply check the level of your car's coolant. It shouldn't empty quickly. However, if it's been a very long time since you've checked, it might be a little low. Either that or you have a leak. 

The simplest version of this situation is that your coolant is just a little low. In that case, you just need to top it off. If you're missing a lot of coolant, however, then you probably have a leak. There is more on how to diagnose and fix this issue further down. 

Click here to shop for engine coolant on Amazon.

2. Malfunctioning Thermostat

Another common cause of this issue is the thermostat inside of your system malfunctioning and staying open. The thermostat's job is to filter coolant into the radiator. If it is stuck open, then it's not doing its job properly. This malfunction causes issues with the heater including the one highlighted in this article. 

To fix your stuck open thermostat you will have to replace it. First, make sure your car is completely cool. Next, locate the thermostat at the base of the radiator. Place a bucket under the radiator hose and then detach it. Now you can replace the broken thermostat with the new one. Put the hose back on and top off your coolant if necessary. The most important part of this job is making sure there isn't air in the system. That can cause major issues down the line. 

How Do You Check If The Thermostat Is Working In Your Car?

To find out if this is the issue, start your car and let it run for 10 minutes or so. Then, shut it off and carefully check the hoses to the radiator. One hose should be hot and one cold. If both hoses are hot, then your thermostat is stuck open.

Click here to shop for thermostats on Amazon.

3. Air In The System

If your car is low on coolant, then there is a good chance that the heating system is air locked. Air gets into the system when the coolant is low for one reason or another. Having your heater only work when the car is moving is a big indicator of this issue. You can purchase a coolant system test kit that will tell you if there is air in the system. A common cause for this issue is simply a bad radiator cap.

To fix this issue, you have to do two things. You have to fix the underlining issue that caused it and you have to bleed the air out. The underlying issue can be a bad radiator cap, low coolant, a leak in the system, and some other things. For a comprehensive guide to bleeding your cooling system, check out the video below:

4. Faulty Heater Valve

The next possible cause for the issue is a malfunctioning heater valve. The heater valve works similarly to the thermostat but allows coolant into the heating core instead of the radiator. To diagnose this issue, allow your car to run for around 10 minutes with the heat on. Then locate the heater valve. It is normally located on the firewall behind the engine. Carefully touch the hoses on either side of the valve. If the engine side hose is hot and the heater side hose is cool, then the heater valve is faulty.

To fix your heater valve, you first have to figure out if it's cable, vacuum, or electronically operated. Figuring out which way it is operated will allow you to test the heating valve corresponding to the operation method. You should be able to adjust it manually, with a vacuum, or electronically to fix the issue. 

5. Radiator Leak

Having a leak in some part of your radiator can cause many of the issues above. If there is a leak somewhere, then your car is losing coolant. As we've seen, that can cause heater issues like air blocking and other negative side effects associated with low coolant. Finding the leak should be fairly easy by just looking. Look for coolant leakage around the radiator and especially where the hoses connect. 

To fix this issue it depends on where the leak is coming from. If there is a crack or another source of damage on your radiator that will probably require a mechanic or a replacement. However, if it is coming from the end of the hose then that's a simple fix. Just tight the clamp holding the hose on and that should fix the issue. 

6. Blocked Heater Core

The heater core is generally tucked behind your dashboard and can have some issues since it's so small. The small holes inside of it can become blocked fairly easily. Blockages are usually caused by old coolant and sometimes dirt and other small debris. 

How Do You Test A Heater Core?

To see if this is the issue, let your car idle for around ten minutes with the heat on. Then carefully check the hoses. Heater core hoses are on the firewall behind the engine. If they are not equally warm then you most likely have some degree of blockage.  

How Do You Unclog a Heater Core?

The simplest way to fix a heater core blockage is to use a flushing agent. These are specially made cleaners that safely enter your cooling system and unclog any blockages. This includes your heater core.

Shop for a coolant flusher on Amazon.

How Expensive Is It To Replace A Heater Core?

It can cost you between $500 to $1,000 to have your heater core replaced. The price can vary where you go but the lion's share of it is due to labor costs. Parts to replace a heater core are relatively cheap. Since the location of the heater core is in a tricky spot behind the dashboard, a lot of the work is just getting to it. 

Is There a Fuse For The Heater In My Car?

Yes, there is a fuse for your car's heater. However, the issue of the heater only working while your car is running is not likely caused by a fuse issue. This is because it controls all electricity going to your heating unit. So if the fuse blows, then your heater wouldn't work at all. Not just when you are idling. 

Final Thoughts

Figuring out the cause of any issue with your car can be tricky. Since your heater issue can be caused by several different things, it's good to know what they are and how to spot them. You can go down the list from most to least likely based on how your car's performing and the things you notice about it. Whatever the determining issue is for the malfunction make sure to be careful and stay safe!

Make sure to click on the links below for more vehicle troubleshooting guides!

RV Furnace Doesn’t Ignite – What To Do? (RV Furnace Troubleshooting Guide)

5.7 Hemi Engine Burning Oil – What To Do?

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