The heater core of a car is an often forgotten but vital component that can cause big repair bills when not cared for properly.

Big repair bills on an automobile are often associated with major parts like the engine, the transmission, or the chassis. Sometimes though, smaller components like the heater core can cause huge problems when not cared for properly. Today, we’re going to break down exactly what a heater core is, how to determine if yours is starting to fail, and when it’s time to call your mechanic.

What Does a Car Heater Core Do?

Person holding a heater core
Person holding a heater core

Think of a heater core in the same way that you might think of a radiator on the front of your vehicle. In fact, to look at them, it might be difficult to tell which is which. The radiator accepts hot coolant from the engine into one side and ultimately cools that same fluid with air rushing in from the atmosphere and through the radiator fins themselves. A heater core does almost the same thing except that it funnels air through to create heat to the cabin.

When you turn on your heater or your defroster in a car with an internal combustion engine, air is funneled through the heater core which itself contains a near constant flow of hot engine coolant. If you’ve ever cranked on your heat first thing in the morning only to get a blast of cold air, here’s why.

Man trying to warm up in his car
Man trying to warm up in his car

When you first start your car, the engine, and consequently, the coolant, isn’t warm yet. So the air passing through the heater core has no heat to gather from it. Once the engine warms up, the coolant is also warm, and the heater core can pass off that heat to the air flow that ultimately exits your climate control vents. Unlike many components that are designed for regular maintenance, the heater core is designed to function indefinitely but sooner or later, almost all of them will fail.

What Are Symptoms Of A Failing Heating Core?

Testing the temperature from air vents
Testing the temperature from air vents

A failing or inoperable heater core won’t set off a check engine light. Instead, you’ll have to diagnose it with other symptoms. If on those cooler days you crank up your heat and it never gets warm, even after 20 minutes or more of driving, there’s a chance that something is wrong. It’s possible that you simply need to add coolant. After all, without enough coolant in your engine, the heater core can’t be full and subsequently can’t pass that heat energy off to the air flowing through your climate control system.

There’s a chance that the heater core itself is to blame. If you’ve only just taken a car out of storage for instance, it’s likely that the seals shriveled up and failed long ago.

For example, if there’s a leak or a clog in your heater core then you’ll still have a lack of heat from the vents. In addition, a leak could explain why coolant levels are low, to begin with. If your car’s engine begins getting hotter than normal or overheating due to a lack of coolant, there’s very likely a leak somewhere. There are other symptoms to look for as well.

Passenger-side footwell
Passenger-side footwell

Most of the time, the heater core is located in the dash towards the passenger side of the car. To that end, when a heater core fails and begins to leak, it’s not uncommon to find fluid, in this case, coolant, pooling up in the passenger-side footwell. Check the carpet for moisture and keep in mind that we’re not talking about the kind you find if you’ve gotten in the car when it’s raining outside. A leaking heater core will usually soak the carpet if given enough time.

It’s possible that before the leak is big enough to soak the carpet you’ll smell a sweet odor in the cabin when the heat is turned on. The heater core could also end up pumping out fog which could indicate failure or a clog in the system.

If signs indicate that you do indeed have a failing heater core, the time to act is right away. Not only is it dangerous to drive your car with a leaking coolant system, but the coolant itself can be harmful if ingested by people or animals.

Should I Replace a Heater Core?

Torn out dashboard -
Torn out dashboard -

Swapping out a heater core is almost always a messy job. Whatever coolant might be left in the core is likely to leak out as it’s removed. In addition, just getting to the heater core often requires the complete removal of the dashboard.

Additionally, in many replacement protocols, the climate control system is dismantled to the point that the air conditioning system will require the complete evacuation of its associated fluid. Once the job is done, it can be recharged for future use.

Professional mechanic working on a car
Professional mechanic working on a car

To put it simply, this is not a job for the backyard mechanic. While some heater cores are easier to access, it can take a lot of work to access and replace the system. We recommend that a heater core replacement is a job for a seasoned mechanic.

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Stephen Rivers

Stephen Rivers is a car enthusiast who loves all things built with passion, extending to nearly all car cultures. After obtaining an occupational studies degree in sports medicine, Stephen turned his attention to sports cars. He was employed as an auto shop manager, spent time in auto sales, and worked as a software developer for a racing company, but Stephen began writing about cars over 10 years ago. When he's not in front of a computer screen, he's racing his own Bugeye Subaru WRX in as many autocross and rallycross competitions as he can.

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