Heater Core


The heater core is a small heat exchanger that engine coolant is circulated through. It is usually located inside the driver's compartment.

A heater core.

When performing any checks and service procedures, always follow the procedures recommended in the manufacturer's service manual.

In most cases, problems with the heating system are problems with the engine's cooling system. Therefore most service work and diagnosis is done to the cooling system.

Coolant flows through the heater core to warm the passenger compartment.

Problems that pertain specifically to the heater are few: the heater control valve and the heater.

  • Most often if these two items are faulty, the engine's cooling system will be negatively affected.
  • Both of these items are replaced rather than repaired.
  • In some cases, it is possible to make repairs to vacuum hoses and electrical connections without removing the heater assembly.

When there is a problem of insufficient heat, begin your diagnosis with a visual inspection and a check of the coolant level.

If the level is correct, turn the heater controls on and run the engine until it reaches normal operating temperature. Then:

  • Measure the temperature of the upper radiator hose.
    • The temperature can be measured with a pyrometer.
    • If one is not available, gently touch the hose.
    • You should not be able to hold the hose long because of the heat.

While doing this, make sure you stay clear of the area around the cooling fan. A spinning fan can chop off your hand.

    • If the temperature of the hose is not within specifications, suspect a faulty thermostat.
  • If the hose was the correct temperature, check the temperature of the two heater hoses.
    • They should both be hot.
    • If only one of the hoses is hot, suspect the heater control valve or a plugged heater core. Like the radiator, heater core tanks, tubes and fins can become clogged over time by rust, scale, and mineral deposits circulated by the coolant.
    • Reverse flushing the core with a power flusher may open up the blockage, but usually the core has to be removed for cleaning or replacement.
  • With cable-operated control valves, check the cable for sticking, slipping (loose mounting bracket) or misadjustment. With valves that are vacuum operated, there should be no vacuum to the valve when the heater is on (except for those that are normally closed and need vacuum to open).
  • Water on the front floor mat or passenger compartment may indicate a leaking heater core. Complete the following diagnostic checks:
    • Check the liquid to see if antifreeze is mixed with the liquid. It should be oily if it contains antifreeze.
    • If it is oily, check all hose connections for leaks.
    • Check the drain passageways to make sure they are not clogged with foreign material.
    • Use a pressure checker to test the pressure in the cooling system. this procedure may detect a small leak in the heater core.
  • Air pockets in the heater core can also interfere with proper coolant circulation. Air pockets form when the coolant level is low or when the cooling system is not properly filled after draining.
  • Flush the cooling system and replace the coolant seasonally.