How Your Heating System Works

It might sound a little funny, but your vehicle's heating system is a part of its cooling system. The cooling system circulates coolant through the engine that absorbs heat. That heated coolant then runs through the radiator, where the heat dissipates. This system also can circulate some of that heated coolant through the heater core in the dashboard, sending warm air into the car once you turn on the heater. The amount of hot air and which direction it blows are controlled by the buttons and switches on your dash, which direct the heater valves and the vehicle's blower fan.

There are many reasons your car's heater isn't working. Maybe the fan is malfunctioning. Perhaps your temperature gauge is running below normal. There could even be a puddle of liquid under the front of the vehicle.

Here's our guide to a poorly operating heater, what can cause it and what to do about it.

Reasons your heating system isn't working

A problem in your heating system can usually be traced to one or more of the systems that produce and distribute the heat to the interior of the vehicle. 

  • The cooling system
  • The heater core
  • The heater valves
  • The blower fan

Get it diagnosed by a professional.

Cooling System Issues

Coolant level:  If your coolant level is low, your heater core may not be getting enough warmed coolant through it to produce adequate heat.

Solution:  Top up the coolant and see if it helps. If the level is low due to a leak, track down the source and get it repaired. A coolant leak could indicate a cracked hose or loose clamp or could stem from a more serious problem like a head gasket leak. Either way, don't wait long to get it fixed — losing coolant is an easy way to overheat and destroy your engine.

And while we're on the subject, it's also important to keep your coolant fresh and clean. If you don't, the helpful additives in the coolant will wear out and other particles will get into it, potentially clogging passages in the heating system. Check your owner's manual for the recommended replacement interval, and ask your mechanic whether your cooling system needs a change and a flush. It's low-cost engine insurance.

Thermostat:  Your 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 reduces emissions. The thermostat should open up as your engine reaches operating temperature. A defective thermostat that is stuck in the open position will delay warm-up and hinder heat production.

A faulty thermostat can also make your temperature gauge read lower than normal and turn your check engine light on.

Solution:  Your mechanic can test the thermostat and replace it if necessary. You may be able to remove the thermostat yourself and test it by boiling it in water and seeing if it closes. But the process can be tricky and involves draining your coolant, as well.

Air lock:  An air lock is a large air bubble that forms in your cooling system as the result of a coolant leak or a recent coolant top-up. An air lock prevents the coolant from circulating properly and can cut your heat output.

Solution:  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 up as necessary. This should bleed off 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.

Bad coolant hoses or loose clamps:  Over time, coolant hoses can deteriorate, become clogged or get totally blocked. Also, the clamps that secure your hoses can loosen over time. A visual inspection of all the coolant hoses and connections will tell you if everything is secure. If you have an older car, check the hoses (with the car off) for a "spongy" feeling — this can mean they're on their last legs.

Solution:  Replace all worn and suspicious-looking hoses (or have them replaced), make sure all clamps are tight, and check for leaks when you are done.

Radiator leak:  This can keep your coolant level too low, especially in older cars. 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 attended to promptly.

Solution:  Your mechanic might be able to repair the radiator, or it may need to be replaced.

Radiator cap:  Your radiator cap regulates the pressure in the cooling system, acting as an escape valve if the pressure gets too high. If it sticks in the open position, there will be insufficient pressure in the system, and the coolant will not get hot enough, reducing the heater's output.

Solution:  Just replace the radiator cap with a new one.

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.

Solution:  Have your mechanic check the water pump to verify its condition. Replacement may be necessary if it's not working right.

Engine fan:  Most vehicles today have a thermostatically controlled electric fan that comes on when additional engine cooling is necessary. A defective thermostatic switch could make the fan run continuously, reducing the coolant temperature to the point where you can't get enough heat into the interior. If the fan runs all the time, including from a cold start, you might have this problem.

Solution:  Getting the thermostatic switch replaced should resolve the problem.

Heater Core Issues

Heater core internal passages:  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.

Solution:  Your mechanic can try flushing the heater core's passages. If this doesn't fix it, a replacement heater core may be needed.

Heater core exterior:  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. This can affect the heater's output.

Solution:  If you can access the heater core, try cleaning the debris from the fins and the air intake passages.

Heater Valve Issues

Heater valves:  These valves control the heat output of the heater core. They can be mechanical or vacuum-operated (like a rotary knob that you turn) or electronic (in electronic climate control systems with specific temperature settings). A valve that is stuck in the closed position will prevent heat from entering the cabin.

Solution for manual valves:  The mechanical or vacuum-operated variety can usually be repaired, with any defective components replaced.

Solution for electronic valves:  The electronic systems are more complicated, as they are usually integrated with the air conditioning system. Some troubleshooting by your mechanic can isolate the cause, which can be mechanical or electrical.

Blower Fan Issues

Blower fan:  If your heater's blower fan isn't working, you won't get much heat from the heater core to circulate into your vehicle.

Solution:  This can be as simple as a blown fuse, it could be a wiring issue, or the blower fan could need replacement. You can check to see if the fuse is blown and replace it, but your mechanic will likely need to intervene if it's more complicated than that.

Information provided by Repair Pal.