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Signs of a Bad Heater Core

The dashboard casing has been removed from this vehicle cabin to expose a bad heater core and a tangle of other automotive components around the steering wheel.

Heater cores are part of the engine cooling system that provides heat to the passenger compartment. Though it’s relatively easy to spot the signs of a bad heater core, the fix is a bit of a headache. Unfortunately, it’s also something that will get worse with time, so it’s worth taking care of as soon as failure is evident. Here’s what you need to know.


Heater cores are essentially mini-radiators. They use the same heat dissipation technology, with coolant flowing through the core to the rest of the system, and getting cooled as it does. The heater core, located just behind the firewall, also serves to bring in heat when you need it. When you turn on your interior heat, a fan behind the core starts up and blows this dissipated heat into the passenger compartment.

Bringing the Heat

The most common problems heater cores develop are leaks. These can be pin-sized and hardly noticeable, or larger in the case of a connecting hose blowing off and spewing coolant everywhere. With smaller leaks, you might see only a tiny trace of coolant on the undercarriage, but if you notice a sizable puddle on the ground or your vehicle develops a voracious appetite for coolant, then your engine is at risk of overheating and you should get the issue repaired ASAP.

Smaller leaks might cause a light mist of warm coolant to blow into the passenger compartment, which will result in incurably foggy windows and a sweet smell. Coolant may also drip inside on the passenger floor once the engine has cooled. The heat supply itself may suffer, but keep in mind this could also be another problem such as a faulty fan or actuator.

Strengthening Your Core

Signs of a bad heater coreReplacing your heater core is, admittedly, a pain. Because it’s buried behind several components on the other side of the firewall, you almost certainly have to remove your entire dash and anything standing in its way, including (depending on the design) panels, speakers and perhaps the steering wheel itself. Newer models packed with sensitive electronics make things even more challenging. Another reason to consider letting the experts at your local NAPA AutoCare handle this repair for you.

Once you get to where you can see it, changing the actual core is pretty simple: Disconnect the inlet and outlet lines and put the new one in … then replace the dash and the rest in reverse order. Don’t forget to refill your coolant reservoir afterward, and don’t mistakenly disconnect the A/C system, which would vent toxic Freon into the atmosphere.

Even if the leak is small, it can take a toll over time — not to mention grow bigger. It’s also probably not great to be breathing in coolant on the regular. If you feel confident tackling the project yourself, pick up a new heater core and replace your faulty one right away. Otherwise, get it taken care of by a professional at your local NAPA Auto Care to protect your engine’s cooling system from the disaster that is overheating.

Check out all the heating & cooling systems parts available on NAPAonline or trust one of our 17,000 NAPA Auto Care locations for routine maintenance and repairs. For more information on the signs of a bad heater core, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo courtesy of Flickr.


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Blair Lampe View All

Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter.  In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.

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