Car Heat Only Works When Driving – Three Main Causes And How To Fix

Many of you might at least once experience car heat only works when driving. On the winter days, cold air at idle must cause you much discomfort and frustration.

So if you are suffering from this disappointing problem, here we have the most straightforward instructions on locating your heating issues and tips on fixing them. Let’s get started!

How Does The Car Heating System Work?

Every car heating system includes five elements:

• A heater core
• A blowing fan
• A heat control valve
• The heater hoses
• A control panel 

Once the engine operates, it will generate heat that travels through the car. The exhaust will release some of it, and the coolant will absorb the remaining portion.

The thermostat will open when the engine reaches its ideal working temperature, allowing the coolant to run through the radiator and engine to release the heat.

The heater core will also warm up the fluid from the radiator. The heat will then be blown to every part of the vehicle thanks to the blower motor in front of the core.

You can adjust the blower speed with the HVAC control panel inside the cabin.

Why Does My Heat Only Work While Driving?

Low Coolant

The car engine eliminates a large amount of heat while operating. The engine will block if this heat is not removed from the vehicle.

So coolant helps to control the amount and flow of heat by collecting and carrying it to your car’s radiator.

A lack of coolant is the main culprit of many heater problems. The solution is frequently found in the simplest response.

Many parts of your car depend on a functional coolant system, so it is usually a red flag when your engine does not heat up while idling.

Your coolant level is the first thing you need to check in this situation. If the coolant level is low, the dashboard of the majority of automobiles will display an alert signal.

Malfunctioning Thermostat

By permitting or preventing coolant flow to the engine, the thermostat in an automobile is in charge of preserving the proper engine temperature. 

It may be unable to close properly due to rust and debris buildup. As a result, whether the automobile is moving or not, the heater may stop pumping hot air.

To check the thermostat’s working condition, you can see if the heater is operating or not when you stop driving.

You’d better check the cabin temperature after a few minutes of driving. If the thermostat is broken, it will not prevent the coolant from flowing to the engine, which leads to incorrect cabin temperature.

Clogged Heater Core

A clogged heater core is not a common problem, yet, it is among the most serious heater issues.

The radiator core warms your automobile’s interior using heated engine coolant from the cooling system.

You might begin to experience these issues with your car heater only works when driving if this component clogs up.

After passing via the hoses, the water pump forces the heated coolant into the heater core. The heater core fins are then moved by the blower motor, causing heated air to escape through the A/C ducts.

Once you switch on the heat, in some cars, a valve opens, allowing hot coolant to travel to the heater core.

The hot air required for your car’s interior is then delivered by this coolant, normally approximately 200 degrees.

Debris accumulation might cause your heater core to become blocked. Additionally, if the cooling engine is not adequately maintained, it can begin to malfunction.

How To Fix Car Heat Only Works When Driving?

1. Low Coolant

Preparation

The only thing you need to deal with the low coolant issue is the new coolant.

Instruction

Step 1. Leave the engine to cool down, disconnect the radiator cap, and insert the funnel into the hole.

Step 2. Fill the reservoir to the top with the fresh coolant. To ensure there is no space, you might need to manually pump the main coolant pipe. Moreover, you should take action carefully to avoid coolant blowing out of the reservoir.

Step 3. Put a new radiator cap on. Once the car has started, see whether the heat turns on.

2. Malfunctioning Thermostat

Preparation

Before starting the work, prepare the following tools and parts: 

• Drain bucket
• Wrenches
• Coolant
• New thermostat

Instruction

Step 1. Give the car 15 to 20 minutes to cool. Now look for the thermostat. It will be between the core and the main hose at the radiator’s base.

Step 2. Lift the flapper valve off and raise the vehicle’s front end for improved clearance.

Step 3. Put the drain bucket under the thermostat and remove the hose to empty the fluid.

Step 4. Take out and swap out the thermostat.

Step 5. Refasten the radiator’s hose and put the reservoir’s lid back on after adding the coolant. Now drop the automobile.

Step 6. Start the car. See if the heat comes on while you wait and drive around for a test.

Step 7. Check to ensure the coolant level has not decreased. If it, unfortunately, drops down, fill it up as needed.

3. Clogged Heater Core

Preparation

Before starting the work, prepare the following tools and parts: 

• Pliers
• Screwdrivers
• Phillip-head
• Work light
• Unclog kit

Instruction

Step 1. Find the heater core and think of the easiest way to locate the supply and return the hoses. It is usually on the firewall. The heater core hose is one of the two hoses that enters the firewall on every car.

Track the two hoses to see whether you can separate them from the engine if you can not reach them through the connector in the firewall.

Step 2. Inspect them carefully to determine the correct instrument to use. If there is not enough light in the dark connecting area, use a work light.

To loosen the hose and pull it out, unscrew the clamps and twist it back and forth.

Step 3. Grab a yard pipe, attach a sprayer, and connect it to a water flow. Then, clean the heater core next.

The pipe’s opposite end will spew out unclean water for you to observe. Change the hose to the different side of the line and rinse it after a few minutes have passed.

Up till clean water is coming from both pipes, keep doing this.

Step 4. Lift a hose, acquire a suggested coolant, and fill it into the pipe until you see liquid pouring out of the other end to reattach the two pipes.

Step 5. If necessary, fill out the coolant in the radiator and reservoir. Switch on the heat after starting the engine and letting it idle for a little while.

Ensure the automobile is not too hot by keeping an eye on the temperature gauge because overheating might make your car unable to start.

Step 6. Last but not least, see if heat is emanating from the heating vents. Tap yourself on the back when warm air is coming out of it.

FAQs

Does The Car Heater Have A Fuse?

Yes, the heater on your automobile does indeed have a fuse. However, it is unlikely that a fuse problem is the root of the problem where your heater only works when driving.

It is because the fuse regulates all power traveling to your heating system. As a result, your heater would not operate if the fuse blew, not only while you are standing still.

How Often Should I Check My Car’s Heater?

One of the customers’ most common problems at the garage for a heating system check is that they hardly check their car’s heater.

So it is highly recommended that you examine your heating system at least once a year.

The process is not very complex. You can finish the checking with the following steps:

• Start your car and the heating system. Leave them there and watch how long the heater needs to heat the cabin.
• Check the car’s undercarriage for leaks and check the coolant level.
• Keep an eye on the thermostat to calculate the time for the temperature to return to normal.
• Watch out for any coolant odors when the heater is on.

Is It Expensive To Replace A Heater Core?

A heater core replacement can cost around $500 and $1000, depending on the core types and the garage you choose.

This high price is usually because of the labor cost since most heater cores locate in quite a tricky place, so the auto mechanic has to work hard to reach the position.

If you can replace the heater yourself, it will be much cheaper. A new core usually accounts for $100 to $300, which is much more affordable compared to paying for a mechanic fix.

Can I Drive My Car If The Heater Is Not Working?

Sometimes, you can still go further when your car has a heater issue. However, driving a car in bad conditions is not advisable because various reasons lead to a broken heater.

Some of the causes might not influence the actual engine. Others, however, can put drivers into dangerous situations if they keep driving, especially a low coolant, broken thermostat, or blocked core.

Therefore, as soon as you discover there is something wrong with your heater, stop driving for a check immediately. Prevention is always better than cure.

Conclusion

Low coolant, broken thermostat, and blocked heater core are among the typical causes of car heat only works when driving.

You can go to a mechanic to check immediately when you discover the problem.

You can follow the above instructions to fix the case yourself. However, you should do the task in a wide flat space like a garage ground, a driveway, or a parking area.