How to Know When a Heater Core is Plugged

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The first telltale sign that you're experiencing problems in your vehicle's heating system is the absence of heat coming from the heat registers. The blower motor is running and you feel air, but it's cold. A plugged heater core may be the culprit, but there are a couple of quick tests you can perform to begin the process of elimination.

Things You'll Need

  • Drain bucket
  • Garden hose
  • Antifreeze
  • Screwdriver or channel locks
  • Check the coolant level in your radiator and overflow tank before starting the vehicle. Low coolant in the radiator can be a major reason the heater core isn't getting enough fluid circulating into it to warm the vehicle. Fill the radiator and overflow tank as needed with equal parts antifreeze and water.

  • Determine whether you're smelling antifreeze, seeing wetness on the passenger side floor or getting a greasy mist on the interior of the windshield when the defroster mode is on the temperature control panel. This may indicate the heater core is leaking and will need to be replaced. It will also keep losing antifreeze until it is replaced.

  • Get the engine up to operating temperature and leave it running in Park or Gear with the parking brake on. Release the hood latch.

  • Locate the temperature gauge on your dashboard and make sure it's running between 190 degrees to 220 degrees (depending on the model). If there are not temperature markings on the gauge, at least make sure it's running in the middle or in the area where it should be. If it's flat, you may have a coolant level problem or a thermostat problem.

  • Open the hood and carefully locate and touch the upper radiator hose to see if it feels warm. Follow the hose to the engine block and carefully touch it down in that area near the thermostat.

  • Locate the two smaller hoses that go into the firewall on the right side (passenger side) of the vehicle. In some vehicles, this may not be all that easy and you may have to raise it and crawl underneath to locate and touch-test them. One of these hoses is an inlet to the heater core coming from the radiator; the other is an outlet coming from the heater core. If the inlet hose is hot/warm and the outlet hose is cold, you can pretty much determine your heater core is clogged.

  • Determine what you want to do. If the heater core hoses are not easily accessible and you have no desire to attempt unclogging the heater core yourself, at least you've isolated the problem and can describe the procedure you performed to a qualified technician. Then he can unclog the heater core for you. If the heater core hoses aren't that difficult to access, you can try to flush out the heater core with a garden hose. Shut off the vehicle and allow it ample time to cool down.

  • Place a drain bucket under the vehicle beneath the hose connections and remove the two hose clamps with a screwdriver or a pair of channel locks (depending in the type of clamps). Note which hose is which for reinstallation, and allow the heater core and hoses to drain thoroughly.

  • Take a garden hose and insert it into the inlet tube. Be careful how much water pressure you turn on in the hose. Depending on your water pressure, some hoses can run up to 100 lbs. per square inch of water pressure--which can easily burst a heater core seam. Add a little more pressure until it blows the gunk out of the heater core. This is messy, so don't be wearing your Sunday best.

  • Replace the hoses to the heater core and tighten the clamps. Add antifreeze to the radiator and start the vehicle again with the radiator cap off in case the system needs to burp. Check underneath for clamp leaks and add antifreeze as necessary until the thermostat gauge in the vehicle gets up to operating temperature. You should feel plenty of heat coming from the heat registers now.

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