How to Check if a Heater Core is Bad

A core leak will be obvious.

A car's heater core is a miniature radiator that has antifreeze running through it. The fan for the heater and the defroster blows across the hot heater core to defrost the windshield and heat the air inside the car. There are many factors that can diminish the efficiency of a car's heating system. In the case of a problem heater core, the signs can be evident.

Instructions

1. 

   Check for leakage. Because of where the heater core is housed, under the dashboard, a leaking core will give you two definitive indications that the core leaks. Antifreeze leaking onto the passenger's floor area is the first. Be sure that the moisture is not the result of a leaking windshield. Use the oily consistency or the color of antifreeze as the determining factor. The second indication is a oily mist on the windshield which appears when the defroster is turned on. As the heater leaks the defroster's fan is blowing the coolant up though the duct work onto the windshield. In some cases a heater hose could be giving you the same symptoms.

2. 

   Locate the heater hoses. Open the hood of the car and look for two black hoses about a inch in diameter that run from the top of the engine back towards passengers' compartment. These will pass through the firewall that separates the engine compartment from where passengers sit.

3. 

   Check for clogging. Once the vehicle has reached operating temperature and the heat has been turned on and the temperature adjustment set to hot, coolant should flow through the heater core. Now that you have located the heater hoses, you can feel the hoses to see if there's a flow of hot liquid through the core. If one hose is hot and the other cold, it's solid evidence that the heater core is plugged up. If both are hot, it doesn't mean that the core is not plugged, it just means there is flow through the heater.

4. Expose the heater core. Because accessing the heater core is such a labor intensive job, it is wise to be fairly certain that the core is the problem. In the case of a suspected clogged heater, the only way to know for sure is to dismantle the heater box and feel the temperature of the core. In the rare case that you can easily access the heater core, bring the vehicle up to operating temperature and feel the core. Keep in mind that it must be hot enough to warm the air that the fan blows against it.

Tips & Warnings

• Be sure the thermostat has opened before testing for a clogged heater.

• Observe due caution while working around an engine at operating temperature.

• Beware of typical cooling system warnings of hot coolant under pressure.