What Are the Signs of a Bad Heater Core?

by Shelley Moore

The heater core is a mini-radiator located behind the dashboard. The fan for the heater and defroster blow across this component to bring heat into the vehicle's interior. Replacing the heater core is something car owners dread because parts and labor are expensive, and it is a difficult do-it-yourself project. Signs of a bad heater core are few, but noticeable.

Other Components Working

If the radiator is full and the thermostat works but no heat comes through the vents, that's a sign of a bad heater core.

Antifreeze Odor

An odor of antifreeze coming from the vents when the heater or defroster is on also points to a bad heater core.

Windshield Fogging

Another sign of a bad heater core is windshield fogging when the heater or defroster is on.

Antifreeze Leak

Antifreeze leaking into the passenger side of the car is a sign the heater core has completely failed.


If the radiator is rapidly leaking coolant into the interior, the vehicle will probably overheat and thus is not safe to drive.

Heater Core Bypass

A mechanic can bypass the heater core by tying off the hoses that connect the core to the coolant system. The vehicle will not have heat inside, but it will still be functional.

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