Bad Heater Core Symptoms

Although the heater core is a simple device, it can cause serious problems when it fails. Replacement can also be very costly due to the part’s location, which is usually behind the dashboard.

How can you tell if your heater core is bad? First, you’ll want to familiarize yourself with the common symptoms of a heater core problem.

5 Signs of a Bad Heater Core

Like your car’s radiator, the heater core is a heat exchanger. But while the radiator transfers heat from the hot coolant into the atmosphere, the heater core transfers heat to the cabin.

You’ll also notice that the heater core, with its tanks and fins, looks just like a mini radiator.

The heater core has an inlet and an outlet pipe that connect to the car’s cooling system. Hot engine coolant enters the heater core through the inlet pipe. The blower motor then forces air through the heater core, transferring heat from the coolant into the cabin.

Note: It’s important to remember that the heater core is part of your car’s cooling system; it relies on hot engine coolant circulated by the water pump.

The heater core is designed to last the life of the vehicle—but that doesn’t always happen. Over time, the device can become clogged or develop leaks. When that happens, you’ll likely notice one or more of the following bad heater core symptoms:

Lack of Heat

As you might guess, a faulty heater core can cause your car’s heater to perform poorly. Anything that impedes coolant flow through the core (i.e., an external leak or internal obstruction) can prevent the core from keeping the cabin toasty.

In some cases, a heater core simply becomes fatigued over time and starts to leak. However, there are also instances where a lack of upkeep can lead to a heater core’s early demise.

Film or Fog on The Windshield

A leaking heater core can also cause the windshield to fog up, as coolant vapor travels through the defrost vents. Try to wipe the fog off of the windshield—if it smears and has a sweet smell, it’s coolant from a leaking heater core.

Coolant Accumulated on the Floorboard

As was mentioned, the heater core is usually located behind your car’s dashboard. If you notice coolant accumulating on the floor of your car, that’s almost a guarantee that the heater core is leaking.

Low Coolant Level

If the heater core leak is severe enough, you may notice a low coolant level when you remove the cooling system pressure cap.

A low coolant level is also associated with intermittent heating. Many drivers complain about a lack of heat when driving even though there’s enough heat when the engine is at idle speed and the vehicle is stopped.

When the coolant level is low, the radiator won’t be able to flow the fluid properly to the heater core.

Reduced engine speed results in slow water pump operation, which lets coolant flow smoothly through the heater core. This process results in heat coming from the heater.

As the engine speed increases, the speed of the water pump also increases. But if there isn’t enough coolant in the system, the water pump will only be able to supply coolant through the engine, which is a path of lower resistance.

Engine Overheating

A low coolant level (caused by a leaking heater core) can lead to overheating and possible engine damage. Even a clogged heater core has the potential to obstruct coolant flow enough to cause overheating.

What Causes a Bad Heater Core?

For instance, if the coolant is not changed at the interval suggested by the vehicle manufacturer, it will eventually break down and become corrosive. When that happens, the coolant begins to eat away at the various metal components—including the heater core—within the cooling system.

Old coolant can also lead to sediment build-up that can clog the heater core.

How to Test a Bad Heater Core

Because the heater core is usually tucked away behind the dashboard, testing it can be tricky. Still, there are a couple of methods you can use to check for heater core leaks and obstruction.

Before testing the heater core, however, you should make sure the rest of the cooling system is working properly. There are a variety of cooling system problems⁠—ranging from a low coolant level to a faulty water pump⁠—that can mimic a bad heater core.

Note: If the temperature gauge has a colder reading than normal, there’s a good chance the thermostat is stuck open, resulting in poor heater performance.

Once you’ve ruled out other cooling system problems, you can test the heater core using an infrared thermometer. If you don’t have access to such a tool, you can perform a less precise version of the test using just your hands.

Here’s how it’s done:

Note: The following information is for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.

1. Start the vehicle and get the engine up to normal operating temperature.
2. Set the heater to the MAX heat position.
3. Check the temperature of the heater core hoses: Both hoses should be too hot to hold. Typically, the inlet hose should be about 20 degrees warmer than the outlet hose.
4. If the outlet heater hose is significantly cooler than the inlet hose, the heater core is probably clogged.
   
   Note: If both heater hoses are cold, the heater control valve (if the vehicle is equipped) may be faulty.
   
   
5. In some cases, you may be able to flush the heater core to fix the problem. But if that doesn’t work, the heater core is likely faulty and should be replaced.

You can also use a cooling system pressure tester to check the heater core (and the rest of the cooling system) for leaks.

Here’s how it’s done:

1. Make sure the engine is cold, then remove the cooling system pressure cap.
   
   Warning: Never remove the cooling system pressure cap when the engine is hot. Severe injury may result.
   
   
2. Top off the cooling system with coolant if needed.
3. Attach the pressure tester in place of the pressure cap.
4. Operate the plunger to pump up the pressure tester to the vehicle manufacturer’s rated system pressure (typically around 14 psi).
   
   Warning: Never pressurize the system beyond the vehicle manufacturer’s specification. If greater pressure is applied, various parts of the cooling system may suffer extensive damage.
   
   
5. If the heater core and the rest of the cooling system are free from leaks, the pressure should hold. On the other hand, if the pressure drops rapidly, there is a leak somewhere.
6. While the system is pressurized, you may be able to spot a heater core leak by looking for coolant under the dash and on the floorboard.

Frequently Asked Questions

Can you drive with a bad heater core?

If you have a bad heater core, you should bring your car in for repairs right away. Driving with a faulty heater core can be risky, as it can lead to overheating and extensive engine damage. Even a clogged heater core can prevent proper coolant circulation, causing your engine to run hot.

But if you must continue driving for a short distance, keep an eye on the temperature gauge. If the gauge begins to climb, pull over immediately and have your vehicle towed to the nearest repair facility.

Can a bad heater core cause overheating?

Yes, a leaking heater core can cause a loss of coolant and subsequent engine overheating. Although not as common, it’s also possible for a heater core to become obstructed enough to restrict coolant flow and cause overheating.

Does a bad heater core affect air conditioning?

In most cases, the answer is no. The heater relies on hot engine coolant, whereas the air conditioning (A/C) system uses refrigerant. The heater and A/C are, for the most part, two separate systems.

You can learn more by checking out our article on the difference between your car’s heater and air conditioner.

What is the typical cost to replace a heater core?

If you choose to have a professional replace your heater core, expect to pay somewhere between $500 and $1,000 to get the job done.

Products Mentioned in this Guide

Heater Core Radiator