How Can You Tell If You Have A Bad Heater Core?

The heater core is one of the components of a heavy-duty or semi truck that many drivers don’t give much thought to – until it goes bad. The heater core is the primary source of heated cabin air, and it also keeps the defroster warm and clears fog from the windshield. Getting into your truck in the cool months, or while in colder areas of the country like Minnesota and the rest of the Upper Midwest, and not being able to get any warm air can be frustrating, as well as a little nerve-wracking. 

But how do you know if it’s the heater core? We’re going to take a look at the various ways you can tell that the heater core in your truck has gone bad, as well as what might cause it to go bad. We’ll also cover how you can test for and potentially diagnose a bad heater core in your truck. In the end, if you’d rather leave the job to an experienced technician, we’ll point you to an expert resource for heavy-duty truck work.


Need truck repair or maintenance? Blaine Brothers semi truck repair shops in Minnesota and Wisconsin help you get back on the road.


How Do You Know Your Heater Core is Bad?

Experienced technicians and truck mechanics will have many different ways of diagnosing and confirming a faulty heater core. However, there are several ways that you can tell with reasonable certainty that the heater core in your truck has gone bad. Here are some of the most common ways you can tell:

  1. Sweet Odors

If you can smell sweet odors either intermittently or constantly while in your cab, it may not be that stash of snacks you have in the back. It could be a strong indication that your heater core is damaged or needs replacing. Coolant has a sweet smell due to the ethylene glycol; some people even describe it as fruity. This is often an indication that your heater core is leaking and the blower is blowing air across the leaking coolant and carrying the smell into the cabin. 

This can be seen as a sign that your heater core hasn’t completely failed yet, but instead is actively beginning to fail. Even the smallest pinhole in the heater core can send a pressurized mist of coolant out into the air. If at all possible, stay away from quick-fix “stop-leak” products, because not only do they rarely fix the problem in any meaningful long-term way, but they also have a significant potential to cause other problems down the line.

  1. Little-to-No Heat in the Cabin

If your truck is running just fine in all other respects – such as normal temperature and voltage – but cannot produce any heat, it may be a sign that the heater core has failed. 

You’ll want to rule out any potential problems with the blower motor by making sure the fuse is still intact and by checking the circuit if needed. If that works, you can eliminate a blend door issue by letting the truck get up to operating temperature, turning the AC on maximum, then turning it to heat. Next, go through the different vent options, like floor, defrost, etc. If one or more aren’t working it’s likely a vent or blend door problem.

Since the heater core is deep in the dash of your truck, getting to it is no simple task, and it requires a significant amount of disassembly. The best thing to do if you have no heat at all is to get in touch with an expert local technician who can put in the work needed to fix your issue with ease.

  1. Coolant Loss

Another sign that your heater core may be going bad is that your truck goes through the coolant at a significant rate. If your truck is constantly in need of coolant to refill the reservoir, the most likely culprit is a leaky heater core. There are situations where the leak may be in hoses or other components but in many cases, it’s the heater core itself.

One way to check if your truck is leaking coolant from the heater core or somewhere else is to fill up the coolant before the next time you plan to park for a day or two and see if there are any leaks that you can spot underneath the truck the next time you go to start it. If you can see a leak from an obvious place, it isn’t the heater core. However, if you see a leak in the cabin or underneath the dashboard, it’s more likely that the heater core is leaking.

  1. Cold Air/Hot Engine Combo

This is the classic way to tell that something is wrong with your heater core. If you can get your vehicle’s temperature up to normal operating levels, turn on the heat full blast, and get only a lukewarm or even chilly breeze coming from the combo, then you know that your heater core has gone bad.

  1. Coolant Fog

There are two ways that a bad heater core can cause your windshield to fog up, and one is far more dangerous than the other. The first way is that your heater core is malfunctioning in such a way that your defrosters aren’t working. The second is that the heater core could have a hole of an undetermined size which is allowing coolant to be pressurized and misted out of the hole.

This mist is then blown into the cabin where it’s either deposited on the windows, dashboard, and other surfaces, or inhaled by the occupant. You can easily tell this apart from conventional condensation or mist, because when it is deposited on a surface it is often difficult to clean off, and leaves a residue.


What Causes a Heater Core to Fail?

Just like any other component on your truck, a heater core can fail for many different reasons. The first and most common is simply age. Low coolant levels can pile additional stress on the heater core and cause more frequent failures. This can have a snowball effect on older heater cores that are weak and nearing the end of their life; when they lose coolant, they deal with more stress, causing them to fail sooner. 


Testing For & Diagnosing a Bad Heater Core

Trying to verify that the heater core is the problem is one of the big challenges because the heater core is buried well behind the dashboard. One of the most popular methods is to get an infrared or laser thermometer to measure the heat levels in the dashboard. If the thermometer can’t pick up the core, it may not be doing its job. 

Lacking an infrared thermometer, you can also test your heater core manually, though it may be more difficult. Locate the heater core hoses, and run the heat on max while your truck is up to temperature. The hoses should be much too hot to hold for more than a second, and if not, your heater core is likely having issues.


Blaine Brothers carries a variety of truck parts, including semi truck heater cores. Shop today!


Partner With Minnesota’s Truck Repair Expert to Keep Your Truck Toasty

If you have noticed any of the symptoms mentioned here in your truck, then it’s probably time to head to Blaine Brothers shop in Minnesota or Wisconsin for a full examination of your heater core and associated components. Making sure your heater core is in good shape isn’t just a problem for winter, as you can also stop worrying about coolant levels and potential overheating problems in summer. Contact us today for more information about getting your heater core replaced in Minnesota or Wisconsin!

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Minneapolis, MN 55449

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Cloquet, MN 55720

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Baldwin, WI 54002



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Columbus, MN 55025