How to Test a Heater Core

by Doug Hewitt
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old engine image by John Sfondilias from

A heater core is part of an automobile cooling system, which keeps the engine from overheating. It can be useful to know how engine coolant flows through the system for heater core troubleshooting. Engine coolant travels from the water pump through a hose and through the heater core, which resembles a small radiator. The coolant exits via another hose and goes through passages inside the engine block, where it picks up heat. It then moves past the thermostat and into the upper part of the radiator. The coolant cools in the radiator, exits via another hose and travels back to the water pump.

Step 1

Turn the engine on, and see if the heater works inside the car. If there is heat coming out of the vents, the heater core is effectively transferring heat from the engine to the air ducts.

Step 2

Check the radiator for coolant and the thermostat for correct operation. You could have a faulty heater core if the radiator is full of coolant, and the thermostat is turning on as the engine gets warm.

Step 3

Smell the flow of air from the defrost vents after the engine has been running for a minute or two. One sign of a faulty heater core is the smell of antifreeze with the heater or defroster turned on. Also, check the floor of the passenger compartment. The foot well on the passenger side can have a small puddle of antifreeze when the heater core has gone bad.

Step 4

Check the air vents, visually, after starting the car engine and turning on the heater. When a heater core has gone bad, you can sometimes see small puffs of smoke.

Step 5

Feel the hoses that go to and from the heater core. If one hose is warm, and the other is cool, the heater core is most likely bad.

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