How To Tell When It's Time To Replace Faulty Furnace Ignitors Or Flame Sensors

by Jenny Banks

Ignoring faulty furnace parts could be costly. Homeowners report that the average cost of routine furnace repairs is between $300 and $400. If you want to eliminate these repair costs from your budget, it can be helpful to know when to replace the faulty furnace parts that can cause more serious problems.

Here are some easy ways to tell when your worn ignitor or flame sensor is in need of replacement.


Modern furnace systems come equipped with an ignitor, unlike older furnace models which featured a pilot light ignition system. Failing to replace an ignitor that has gone bad could compromise the effectiveness of your furnace.

Here are two simple systems you can use to test your ignitor:

  • Reset the ignitor. Your furnace's ignitor can fail to function properly while still being operational. To determine if it is time for a replacement, you can reset the ignitor. To accomplish this task you simply need to locate the ignitor and switch it to the "off" position. After waiting 5 minutes, turn the ignitor back on and set your thermostat temperature high enough to kick your furnace on. If heat is produced, your ignitor still works. If no heat can be felt, it is time to invest in a new ignitor.  
  • Test the ignitor's voltage. To determine whether or not your ignitor is still generating the electricity needed to fire up your furnace, you can use a voltmeter. All you need to do is disconnect the ignitor from your furnace and attach the leads from the voltmeter to the ignitor's plug. The voltmeter will read 120 volts if the ignitor is functioning properly. If your reading returns less than 120 volts, replace the ignitor.

Flame Sensor

The flame sensor plays an important role in maintaining the safety of your furnace. If you find that your furnace is not producing as much heat as you'd like, here are 2 things you can do to determine if the problem is a faulty flame sensor:

  • Use a light source to test the flame sensor. Remove the flame sensor from your furnace and attach it to an ohmmeter (a device that measures electrical resistance). Hold the flame sensor near a light source. If the ohmmeter returns a reading of low, you know your flame sensor is still working. Any other reading indicates a faulty flame sensor that should be replaced.  
  • Use your body heat to test the flame sensor. By attaching the flame sensor to an ohmmeter you can use your body heat to test whether or not the flame sensor still works. Grasp the sensor in your hand for 10 seconds. Once you release the sensor the ohmmeter should return a reading of high. If the reading does not change, replace your flame sensor.

Learning to recognize when furnace parts have gone bad will not only allow you to save money on repair costs, it will allow you to replace faulty parts before they turn your furnace into a safety hazard within your home. For more information, or if you need to purchase new furnace parts, visit