Car Heater Only Works When Driving – How To Fix!

You pull up to a stop in your car on a cold day, and suddenly your cabin’s heat turns into an icy blast. What gives? Stay warm this season by learning what could be causing your heater to only work when you drive and what you can do about it.

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

  • Low coolant
  • Malfunctioning thermostat
  • Air in the system
  • Faulty heater valve
  • Radiator leak
  • 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!

Why Your Car Heater Only Works When You Driving The Car?

Your car’s heater works in tandem with your engine’s cooling system. Many issues with inconsistent heat in the cabin can stem from coolant-related issues. If your car only has heat when driving, the following issues may be to blame.

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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.

1. Low Coolant

Coolant regulates engine temperature and the flow of hot air into the cabin. Your engine accumulates a lot of heat when working, and coolant helps to collect this generated heat by transporting it to your vehicle’s radiator for cooling.

If the coolant is low and your heater is on, you may notice that your car heater only works when driving.

A properly functioning coolant system is essential to many components within your vehicle, and having no heat at idle is typically a sign that something is amiss. The first thing you should check in this instance is your coolant level.

Caution: Never open a perceived hot or pressurized cooling system. Doing so could lead to personal injury. Check your owner’s manual for how to check coolant levels or have a professional check the cooling system for you.

There are many reasons for low coolant levels, including irregular maintenance and leaks within the coolant system.

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 houses are hot, then your thermostat is stuck open.

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.