Will the heater core leak if the heat is off? It may be possible

Located under the dashboard of a car is the heater core. This device warms up a car’s interior, using hot coolant from the engine.

Unfortunately, it’s prone to punctures or small holes, which can be caused by many things including ionization-triggered chemical changes. Unfortunately, a punctured heater core will leak coolant, putting your engine at significant risk.

Some motorists will not hesitate to turn off their heater core if it’s leaking. But is this solution effective?

In other words, will the heater core leak if the heater is off?

The answer depends on whether the heater core is equipped with a control valve or not. If it is, the heater will stop leaking if turned off. However, it’s not, it will continue to leak even if turned off unless you bypass it.

Don’t worry, I’ll show you how to do that in a bit.

Possible Cause Why a Heater Could Leak If Turned Off

Some heater cores come with a control valve that allows or prevents hot coolant from getting into the heater. This valve works on its own, depending on the information it receives from the sensor and the computer.

If the sensor detects a problem with the heater, it relays this information to the ECM, which in turn triggers both the heater and the control valve to shut off. This ensures that there’s no coolant coming into the heater, preventing a leakage.

Other heater cores don’t have this intuitive feature. As such, coolant continues to flow into them even if they are off, leading to leakage.

How To Bypass a Leaking a Heater Core

To bypass the heater core so that coolant doesn’t get into it and consequently leak;

  • Open your hood.
  • Remove the two black hoses at the rear of the engine going into the firewall from the heater core, putting a tight fitting pipe between them.


  • Go to your car’s fuse box.
  • Pull out the fuse labeled “Heater.” N/B: It shouldn’t be hard to locate this particular fuse since every fuse in the box is labeled.

Any of these methods will turn off the heater while allowing the coolant to continue circulating even though away from the heater core. The best part is that any of them will not affect the performance of your engine, at all.

How Do I Know If My Heater Core Is Leaking?

You know your heater core is leaking if;

  • Your heater core is not working.
  • The smell of the coolant is coming through your vents. Actually, this fluid has a fruity, maple, candy, or syrup smell.
  • The coolant is dripping down from under the dashboard where the heater core is located.
  • Your windows are fogged out as a result of the fog created in the cabin by the hot coolant.
  • The car cabin is cold due to an insufficient amount of hot coolant to warm the air, thanks to the leakage.


You can think of your car’s heater core as the opposite of a radiator in reverse. The device’s main role is to heat the air passing through it instead of cooling it, using hot coolant from the engine.

Should it develop a hole or puncture and start leaking, there are two ways to fix the leakage, depending on the type of heater.

If the heater is equipped with a control valve, all you need to do is turn it off. This will stop coolant from getting into it. However, if it’s not, you’ll have to bypass it, using any of the two methods above. Similarly, this will keep the hot coolant away from it.

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