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  • Writer's pictureMaestro Truck & Auto Service

Car Heater Issues?

It's that time of year... when we may have to use the car heater from time to time... The worst feeling is getting in your car, turning on the heater and finding it’s not working!

So, how does your heating system work? The heating system is part of the vehicle’s cooling system – yes, that sounds silly – but it’s true. The cooling system circulates coolant through the engine that absorbs heat, that heated coolant then runs through the heater core, sending warm air to the car when you turn on the heater. The amount of hot air and the direction it blows are controlled by the button and switches on your dash – which direct the heater valves and vehicle’s blower fan.

Ok, now that we know how it works, what are some reasons the vehicle’s heater may not work? There may be several reasons for a problem in your heating system but can usually be traced to one or more of the systems that produce and distribute the heat to the inside of the vehicle.


- Coolant Level

Your coolant level can affect the performance of your cooling system. If your coolant level is low, your heater core may not be getting enough warmed coolant through it to produce enough heat.

Top off the coolant and see if it helps. A coolant leak may be the reason for low level coolant – this can indicate a cracked hose or loose clamp, but may also stem from a more serious problem like a head gasket leak. Regardless the reason, you should never wait to get it fixed – losing coolant is an easy way to overheat and destroy your engine.

- Air Lock

An air lock is a large air bubble that forms in your cooling system as the result of a coolant leak or recent coolant top off. An air lock prevents the coolant from circulating adequately and can reduce the heat output.

To correct this, you should set the heater to its maximum setting, remove the coolant tank cap and fill it to the proper level. Start the engine with the cap still open and let the engine idle for a few minutes. If the coolant level should drop as the thermostat opens, top it off as needed. This should resolve the air bubble. When the engine is fully warmed up, put the cap back on and take a drive to see if the heat output has returned.

- Thermostat

The thermostat is a valve in your cooling system that stays closed when the engine is cold, creating a shorter coolant circulation path. This makes the engine warm up more quickly and reduce emissions. The thermostat should open as your engine reaches operating temperature. A defective thermostat that is stuck in the open position will delay warm-up and affect heat production. A faulty thermostat can also make your temperature gauge read lower than normal resulting in your check engine light coming on.

The thermostat should be tested and replaced if necessary, although you can remove and test the thermostat yourself, the process can be tricky and involves draining the coolant – so it may be a better idea to have your mechanic perform this work.

- Coolant hoses or loose clamps

Over time, hoses can deteriorate, become clogged or get blocked. Clamps that secure hoses can also loosen over time. A visual inspection of the coolant hoses and connections will tell you if everything is secure. If you have an older vehicle, check the hoses (with the car off) for a ‘spongy’ feeling – this may indicate a replacement is needed. We recommend replacing all worn and suspicious-looking hoses, making sure the clamps are tight and checking for leaks.

- Radiator Leak

A radiator leak can be the cause of a low coolant level. Look for puddles of coolant under the front of your vehicle. You may also find a dripping or wet area on the radiator. A bad radiator should be repaired or replaced immediately.

- Water Pump

The water pump circulates the coolant throughout the engine and heater core. On older vehicles, it can be a source of leaks and inadequate coolant circulation. Your mechanic should check the water pump to verify its condition and replace if necessary.

- Engine Fan

Most vehicles have a thermostatically controlled electric fan that comes on when additional engine cooling is necessary. A defective thermostatic switch can make the fac run continuously, reducing the coolant temperature to the point where you can’t get enough heat into the interior of the vehicle. Your mechanic should check the condition of the thermostatic switch and replace if necessary.


- Heater Core

The heater core is like a miniature radiator built into the dashboard. It gathers heat from the warm coolant that circulates through it. But its narrow passages can become clogged from rust particles or other contaminants that can build when the coolant doesn’t get replaced or the cooling system doesn’t get flushed for a long time. Your mechanic can try flushing the heater’s core passages.

A second issue could be the heat-radiating fins on the outside of your heater core could also be clogged with debris that makes its way in from the outside air intake at the base of the windshield.

If flushing the heater core’s passage and/or cleaning the debris from the fins doesn’t fix the issue, a replacement heater core may be needed.


- The heater valves control the heat output of the heater core. They may be mechanical, vacuum-operated or electronic. If the valve is stuck in the closed position it will prevent heat from entering the vehicle cabin. Your mechanic should be able to determine the cause for failure and implement a solution based on the valve type.


- If the heater’s blower fan isn’t working, heat from the heater core will be unable to circulate into your vehicle. Your mechanic will need to determine if the cause is a blown fuse, wiring issue or if the blower needs to be replaced.

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