How to Fix a Heater Core Leak

by Shelley Moore
car heater vent image by robert mobley from

The heater core functions as a mini-radiator to bring heat into the vehicle's interior as the heater and defroster fan blows air across it. When heater cores go bad, they leak antifreeze onto the floor of the front passenger area. Replacing a bad heater core is a complicated do-it-yourself project, but having a mechanic do the work can cost several hundred dollars. You can work around the eventual replacement if you cannot afford a new one and need a temporary fix.

Step 1

Add a can of radiator stop-leak additive to the radiator (see Resources).

Step 2

Run the engine for a few minutes to distribute the fluid throughout the system.

Step 3

Disconnect the hoses attached to the heater core if the stop-leak additive didn't fix the leak and the hose clamps are accessible .

Step 4

Connect the hoses to each other with a 5/8-inch or 3/4-inch plastic hose connector or hose coupler.

Cut the hoses at the point of entry if the hose clamps are not accessible. Connect them with the hose connector or coupler.


  • Do not work with the system when the radiator is hot.
  • Don't drive the car for any distance until you are sure you have fixed the problem.

Items you will need

  • Radiator stop-leak additive
  • Hose repair kit
  • Utility knife

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