What are the Signs of a Bad Heater Core?

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Imagine this: It’s winter, you’re carrying all your groceries, desperate to rush into the safety of your car to escape the chilly air. You hastily unlock your doors, fumbling with your keys as your gloves get in the way. Once you successfully open the doors and set your groceries down, you let out a sigh of relief as you enter the inviting cocoon of warmth that is your car, expecting the heaters to save you from the dastardly cold confines of winter’s hug.

After about two minutes of waiting for the warm air to touch your face, a sudden realization hits—the air’s cold, too!

The heater core helps transfer heat from the hot engine coolant into the cabin.

The heater in your car has failed you. What happened? To answer this, we need to know the parts of the heater. The primary components within the heating system include:

If your heater isn’t working, it’s probably because one of these components has failed…

What is a Heater Core?

A heater core is like a smaller radiator that is also part of the cooling system of your car. However, unlike a radiator, which removes heat and disperses it into the atmosphere to keep the engine from overheating, the heater core does the opposite. It still takes heat away from the hot engine coolant, but it uses this heat with the help of some air from the blower motor to pump warm air into your cabin through the air vents.

Bad Heater Core Symptoms: What Happens When the Heater Core Goes Out?

What’s the worst that could happen? So you’ll have to wear more layers when you’re in your car, and maybe you have to keep your scarf on and wear extra thick gloves while driving—not a big deal, right? But the thing is, a bad heater core doesn’t just mean driving in a cold car. There are worse things that could happen to you if you leave the heater core in its current state.

These are some of the most common symptoms that will alert you to heater core problems:

Inoperative Heater

Over time, the heater core can become clogged with sediment from the cooling system. In this scenario, you won’t have any leaks but you will have an inoperative heater.

Slimy Film Build-Up on the Windows

You know those fog patches that form on your windows when the temperature outside is much colder than your temperature inside? You may notice a similar phenomenon when the heater core fails—only this time, the fog will be made up of slimy coolant. Aside from compromising your visibility, the fogging is a sign that your heater core is leaking.

And that’s a problem that could lead to costly engine overheating.

window fogging is a sign of a bad heater core
You may notice fogging on your windows if the heater core is leaking. The fog will be made up of slimy coolant.

A Sweet Smell Suddenly Fills Your Car

As stated above, the sudden fogging often comes with a sweet smell that suddenly fills your cabin. This fruity, sweet smell is coolant, which isn’t good for you to breathe in.

Overheating Even When it’s Cold Inside

If your heater core is leaking, you’re losing engine coolant. The engine must remain full of coolant to prevent overheating. So, you can do the math: a leaking heater core may cause engine overheating.

Sudden fogging and your car overheating are serious problems that may cause harm to both you and your vehicle. Visit your mechanic as soon as you can if you notice any of these symptoms. This is important to note because the warning light nor the temperature gauge may not light up, especially during a road trip or while your car is moving.

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 mechanic.

What Happens if I Bypass the Heater Core?

Replacing a heater core can be labor-intensive—and costly. As such, some people are tempted to bypass a leaking heater core. Usually, they do this by connecting the two heater hoses, either with a length of tubing or a dedicated heater core bypass kit.

Typically, if the bypass is done right (i.e. it doesn’t leak), it won’t hurt your engine. But some argue that a bypass can damage your engine during an overheat situation. That’s because, to lower your engine’s temperature, some experts recommend turning on your car’s heater. The action helps transfer heat from the engine into the cabin.

Bypassing the heater core is highly discouraged. Problems WILL come up if you decide to do so.

Obviously, if you don’t have a heater core, that trick won’t work. But let’s be honest: If your engine is overheating to the point where you have to turn on the heater, you’ve got bigger problems—and you shouldn’t be driving your car at all. Instead, you should pull over (with the engine off) and wait for a tow truck.

Still, that doesn’t make a bypass a good idea. If you bypass your heater core, you still won’t have heat. And your defroster won’t work well, either, which is a safety concern.

To summarize: Bypassing your heater core is a no-no. Unless the bypass is a temporary measure used in an emergency situation, it should be avoided altogether. The only option is to fix the problem properly by replacing the heater core.

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Staff Writers

In the Garage with is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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