What's Wrong With My Car's Heater?

Colder days are on their way, so it's time to make sure your car's heater is pumping out all the hot air you need. Here's what to do if your vehicle is struggling to turn up the heat.

In the recently chilly mornings I've noticed my heater isn't putting out hot air. With winter approaching, I'd like to get this sorted out before the cold weather turns me into a commute-cicle. How do I get my car heater working again?

The method used to warm a car's cabin isn't a whole lot different from the one used to keep its engine cool. In fact, it's all hooked together and runs on the same circuit. Coolant circulates through the engine and absorbs engine heat, which is exchanged with the outside air by way of the radiator. A much smaller radiator called the heater core uses the same hot coolant to keep the cabin toasty. In general, there are two things that can go wrong with this setup.

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The first item to check is the engine temperature. To help cold engines warm up fast, the flow of coolant is restricted by way of the thermostat thermal valve that opens when it's hot and closes when it's cold. When the thermostat wears out, it remains stuck open or shut, which leads to either overheating or cool operation. A worn-out thermostat might be preventing the coolant from getting warm enough to heat the cabin. Replacing the thermostat can be a 20-minute job or a real hassle. Check your service manual to see how involved the replacement is.

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If the thermostat is fine, the flow of coolant in the heater core may be restricted by built-up goop. Sediment and grime can accumulate between coolant changes and collect in the heater core. A flush can be done by disconnecting the heater hoses at the water pump (when the car is cold) and using compressed air to push the coolant and gunk backward, out the inlet hose. Be sure to capture all the old coolant and dispose of it properly. Follow that with a few rounds of filling the core with tap water and flushing it out in the same way, then refill the core with mixed coolant and reattach the hoses. None of these fixes is hard, but the heater-core flush is messy, so you might want to take your car to a shop for service.

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