Auto Repair Q&A



Cooling System










Auto Repair Products

Diagnosing Heater Core Issues

Heater Core (manual drawing)

Now that winter is blustering through the country, it's a good time to talk about heater cores again. They always seem to fail right about when they're needed the most and when it's least convenient to fix them. Lets go over the general diagnostics you should use to quickly find what is wrong with your heater core before you start tearing down your dashboard to replace it.

Good Signs It's the Heater Core

Leaking liquid, if you can get to it and find out whether it's engine coolant ("anti-freeze"), is a very good sign that your heater core or the hoses running to it have ruptured. The trick is to make sure that it's engine coolant and not something else making the floor wet.

Other problems like condensation, a leaking windshield, or even just spilled soda in the glove box are other things that can make liquid leak to the floor under the dash. Use a container to catch some (if possible) or a cotton swab or cotton ball to soak some up. Cotton is a good choice because the white will let you see the color of the liquid. If it's the same color as your anti-freeze, you know what it is. If not, it may be something else. A sniff test could help. DO NOT TASTE IT.

If you still aren't sure or if it's not anti-freeze on the cotton, then you will be looking for some other problem, such as a windshield leak or condensation point.

No Heat In Cabin

If your problem is that you aren't getting heat into the cabin, but the fan seems to be running, then your issue could be one of several things. Assuming the fan runs and air blows through the vents, you can rule out the blower as the problem.

Begin by checking the climate controls themselves. Turn on the air conditioning and see if cold air blows once it's running. If so, you don't have a problem with delivery.

Now check the heater core by warming up the engine to operating temperature and turning on the heat. Feel the hoses entering the firewall at the back of the engine which feed hot coolant to the heater core. If both are hot, you are delivering and returning hot water and the core is probably fine. If you cannot safely reach those hoses (even with the engine off, once it's heated up and the heater is on), try going underneath the dash to access the heater core's compartment, which may allow you access to those hoses without removing anything from the car.

If the heater core is getting and returning hot coolant, the core is not your issue. You probably have problems with the louvers that control airflow into and out of the heater core's box.

Replace the Core

If the problem comes down to the heater core being bad, then replacement is the obvious choice. This is a long, tedious job on most vehicles and requires thorough disassembly of a large portion of the dashboard to access the core for removal. If your problem is leaks, find out where the leak is before you make any replacements to save time and money.