Why is My Heat Not Working in My Truck?

One of the worst feelings when you drive a heavy-duty truck for a living is getting into that truck, starting it up, and cranking up the heat only to be disappointed by the temperature of the air or the amount of warm air getting into the cabin. This can make driving your truck uncomfortable, and it can be unsafe as well, especially when you’re driving through Minnesota and the rest of the Upper Midwest in the winter. Without heat, you can’t defrost your windshield and keep fog and condensation at bay. 

We’re going to look at many of the reasons that the heat in your heavy-duty or semi truck may not be working. For each one, we’ll also dig into any potential fixes, solutions, or what you’ll need to repair the issue. In the end, we’ll also point you towards a trusted heavy duty truck repair shop to help get the heat back on in your rig.


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


The Heating System in Your Truck

Before getting into what possible problems you may be facing with the heating system in your truck, let’s take some time to understand how it functions. So many people drive trucks but don’t understand some of the more basic systems in them. The heat in a truck is relatively simple.

The engine produces heat, and it produces lots of it. Coolant is pumped in and around the engine to absorb a large portion of this heat. This coolant is eventually routed through hoses away from the engine and into the heater core. The heater core is the primary component responsible for creating heat in your cabin and keeping your defroster working. No matter where in the country you live, your heater core is an essential component in the operation of your truck.

While the coolant is in the heater core, the excess heat that it had absorbed from the engine can be dispersed more easily, while also creating a heating source for the cabin climate control and defroster. The heated coolant in the heater core acts as a large heating element, and the blower moves large amounts of air across the heater core, sending that warm air through the vents and into the cabin.

As the coolant is continuously pumped in and around the engine, it keeps the heater core extremely hot at all times. This means that when you turn on your heat and the blower motor starts moving air, you should feel warm air almost instantly. If you don’t, it means that there is an issue with one or more of the components involved in your heating system.


Man inspecting vehicle radiator

Why Isn’t My Heat Working?

There are so many reasons that the heat may stop working in your heavy-duty or semi truck, but some of the most common are listed below. Some issues are much simpler than others, not only to diagnose but to fix. Some can be done by the average DIYer in a short time, while others will require a properly-equipped shop and an experienced technician to fix. Here are some of the most common reasons that you may find the heat not working in your truck:

  1. Thermostat

One of the most common reasons that heat in a truck stops working is that the thermostat is stuck open. While most thermostats are a part that only costs a relatively small amount, they are an incredibly important part of the overall operation of the engine, and they can have an immediate cooling effect on the heating abilities of the truck if stuck open. When stuck open, thermostats allow the coolant to continue freely circulating, never allowing much heat to build up, and often never allowing the truck to heat up to proper operating temperature. 

A thermostat can be one of the easiest DIY fixes for a truck that has no heat. The process is simple, the tools needed are minimal, and the thermostat and coolant can be obtained from any local parts supplier. In addition to the part and the coolant, you’ll need a socket set and a drain bucket. Make sure you’re doing this on a flat, level surface like a garage floor or driveway, and that you won’t be violating any municipal codes by working on your truck out in the open.

If the truck was recently driven, first allow it to cool sufficiently. Locate the thermostat, placed somewhere between the radiator core and the main hose, often near the base of the radiator. Block the wheels and lift the front of the truck, placing the drain bucket underneath the radiator core. Next, remove the radiator hose and let the coolant drain.

Now you can remove the old thermostat and replace it with the new one. After that, reattach the radiator hose and add coolant to the system, replacing the cap when finished. Lower the truck, start it and let it come up to temperature before testing the heat. After a short test drive, check the coolant again, and top it off.

  1. Air Bubbles in the Coolant System

Many people that try to refill their coolant find that they end up with air bubbles in their coolant system or hoses. This is often because when the radiator was refilled with coolant, it had some air gaps in it that were not purged properly. This normally doesn’t have too much of an effect on the performance of the heat in your truck, unless there are a lot of air bubbles. These air bubbles prevent the coolant from circulating consistently throughout the system and can lead to premature wear on pumps and other components. 

Sometimes, this can happen because the radiator cap itself is faulty. Other cases, however, see it happen as the result of having the truck serviced somewhere that doesn’t top off the coolant properly after flushing the system or core. If there are air bubbles in your truck’s radiator, you will need to have the system “burped” or purged of the excess air. Depending on your coolant system this can be a relatively complex process that involves activating bleeder valves in sequence to ensure no air is left in the system. The best solution is to take your truck to an experienced technician so that you can be sure it’s done properly.

  1. Blend Doors & Vents

Blend door actuators and vent actuators are common components that go bad and need to be replaced. Blend doors are the doors that direct the hot and cold airflow to the floor, dash, and so on. They are moved and controlled by the blend door actuators, which are just small parts that move the doors back and forth to match the operator’s preference. When they stop working they may fail in a position that largely blocks airflow to the cabin, leaving you stuck in the cold.

  1. Faulty Controls

The physical controls for your climate control systems – the dials, buttons, or switches – can go bad just like any other electrical component. In some cases, these controls can be accessed and tested with little hassle, though in other situations the dashboard needs to be partially disassembled in order to get to the controls. However, if you can get to them, you can test them relatively easily with a multimeter. If this sounds like more of a project than you are ready to get into, or if you aren’t sure that the controls are the problem, it might be best to partner with a local technician to have a professional diagnose the issue.


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


  1. Low Coolant

The heat in your truck depends on the coolant carrying engine heat to the heater core, where the blower or fan moves it through the vents into the cabin. If there is a low coolant level in the system, however, not only is there less heat in the heater core to be transferred to the cabin but there is also less coolant to absorb heat from the engine. Low coolant levels also add potentially damaging stress to the operation of the heater core.

  1. Blown Fuses

Blown fuses are another incredibly common reason that the heat isn’t working in your truck, and it’s one of the easiest to troubleshoot and confirm. All you need to do is look up which fuses are related to your coolant system and climate control, which you can generally find in the owner’s manual for your truck, and ensure none of them are blown. To do this, take them out one at a time, inspect the fuse, and replace it before moving on. This can be a quick and cheap fix for a no-heat situation, and it can save a lot of hassle to check fuses before taking your truck off the road for a service appointment. 

  1. Wiring Issues

Wiring issues can be one of the most difficult problems to fix in a truck, and they can take the longest to troubleshoot and diagnose. If you are having issues with the wiring in your truck, do not try to fix it yourself. Make sure you leave it to an experienced professional that can use the proper techniques to not only find the problems but to fix them. In some cases, repairs are possible, while in other situations the system may need to be rewired entirely.

  1. winter semi-truck driving and prevention of breakdownsHeater Core Problems

The heater core is arguably the most crucial part or component to getting heat in your truck, and there is more than one way that it can fail. One way is by simply getting clogged with debris and sludge, while the other way involves becoming damaged or eventually succumbing to wear and age. Both ways can be fixed, with various costs and expected life spans resulting from the repairs. 

One of the common ways in which a heater core fails is that it becomes clogged. This restricts the flow of the coolant either completely, or to a largely ineffective trickle. This reduced flow doesn’t allow enough heat to build up in the heater core, which results in little to no heat being sent into the cabin. The solution to this is to get access to the heater core hoses and connect them to water supplies to flush the heater core out with water pressure. This will generally clean the core out enough to facilitate normal operation again for quite some time. Extreme cases may require core replacement.

The other common way for heater cores to fail is to simply break or become too old and weak to stand up to the pressure of use. This generally starts with a small pinhole that corrodes into the heater core housing, subsequently allowing a fine mist of coolant to spray out when the heater core is being used. This coolant accelerates the corrosion, and the lower coolant level in the heater core makes it more difficult for the core to stand up to the stresses of use. Heater cores cannot be repaired, so the only option is a replacement.


Blaine Brothers: Minnesota’s Expert in Truck Heat Repair

If you’re driving through Minnesota and your truck’s heat has been acting up lately, or if it’s gone out entirely, don’t wait to get it fixed. No heat in your rig makes for an incredibly uncomfortable ride and not being able to defrost your windows and keep the fog down can be dangerous as well. If you’re ready to let a trusted local technician in Minnesota or Wisconsin take a look at it, reach out to Blaine Brothers today, and let us know how we can help!

Blaine, MN


10011 Xylite Street NE
Minneapolis, MN 55449

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1325 Hwy 45
Cloquet, MN 55720

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750 Heaton BLVD
Clearwater, MN 55320

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2500 Alreich Ave
Baldwin, WI 54002



9515 150th Avenue NE
Columbus, MN 55025