Why Does My Car Have No Heat?

Man repairing car
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It's the dead of winter and you go out to your garage to warm up your car before heading to work—only to discover that you have no heat. Not only will your drive be cold and miserable, your teeth chattering and your hands near-frozen on the wheel, it might also be dangerous. Because if your car's heater doesn't work, your defrosters won't, either. Which means you'll likely spend the trip trying to wipe the frost off the inside of your windshield with a towel while driving.

But before you do that, see if you can troubleshoot the problem first. There are a number of things that can cause your heater to fail. It could be that heated coolant isn't reaching the heater core for some reason or that the fan that blows the hot air coming from the core is bad.

How the Heating System Works

Your car's heating system is not all that complicated. Your engine runs by turning a mixture of fuel and air into a series of tiny explosions, so naturally, this process generates a lot of heat—so much so, that your vehicle must also have a cooling system to prevent it from overheating. That system consists of a water pump to circulate a 50-50 mixture of coolant and water through the engine, a radiator to release some of the heat into the air, a thermostat to decide when you need to cool your engine and when you don't, and coolant—that colored liquid that circulates through the entire system.

When that coolant heats up, it is transferred into your vehicle's heater core via a few feet of high-temperature rubber hose. The heater core works like a small radiator: A fan blows air over the heated fins of the heater core, creating heated air, which in turn is transferred through a series of vents into the cab of your vehicle to warm its interior.

Check the Core

Your first troubleshooting step, then, is to determine if the heater core is filling up with the hot coolant necessary to start the heat transfer process. This is easy. Simply wait until you are driving at a good speed—say, 40 mph or faster—and switch the controls to heat. If you feel even a ​tiny trickle of warmth coming through the vents, then you know that your heater core is likely receiving hot coolant. Which means the problem probably lies with your heater fan. Check your controls to be sure your fan is on, and try it on different fan speeds to see if you just have a dead speed. If you do, then change the fuses to the fan and you should be up and running.

Check the Coolant and Water Pump

If you don't feel even a trickle of heat, and your fan operates at all levels, it might be that your heater core is not receiving hot coolant once the engine heats up. First, check your coolant level to be sure there is enough liquid in there to even get to your heater core. If your radiator is very low on coolant, you'll get no heat. If your levels are okay, you either have a bad water pump or a thermostat that isn't opening.

Check the Thermostat

If your car has not been overheating or running hot, your water pump is not the culprit. Instead, you might have a failed thermostat, which opens and closes circuits in the cooling system as the engine heats up. If a thermostat is stuck in the closed position, it will never allow coolant to circulate fully, so no heat for you. You can fix this problem by removing the lower radiator hose and installing a new thermostat. Consult your repair manual for details on how to do this.

If you don't have the time or inclination to trace the cause of your failed heat, a shop can do it for you—often the repairs for this kind of problem are very reasonable.

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