Lennox Furnace Troubleshooting: Common Problems and Solutions

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Last updated: 
June 19, 2018

Lennox furnaces are quality units; however, similar to other furnaces, after some time, they can encounter their share of issues due to normal wear and tear or a lack of regular care and maintenance. When this occurs, you may notice that your furnace just doesn't perform how it used to. Fortunately, many issues can be pinpointed and, in some cases, even repaired yourself.

When Lennox furnace troubleshooting, some of the most common issues include:

Lennox furnace troubleshooting

Lennox Furnace - Common Issues

The Furnace Won't Start

If your furnace just stopped working, it could indicate an issue with the thermostat. Sometimes, either the thermostat has been switched off accidentally, or the thermostat has completely gone out. This can cause the unit not to come on. In this case it will need to be exchanged for a new one. Sometimes a circuit may have tripped, which can also cause the unit not to start.

The Furnace Is Not Heating

If you arrive home or arise in the morning to a cold house, it could be that the thermostat is set on a low setting, which can cause the furnace not to put out heat.

If the igniter fails, it will also cause the furnace not to produce heat. A dirty or faulty pressure switch, which monitors the burner to detect a flame, can also cause the furnace not to produce heat. If the flame sensor does not detect a flame, as a safety precaution, the control board will shut off power to the gas valve to prevent the furnace from heating.

The Furnace Won't Stay On

If the furnace fires up but then shuts off after a few minutes, it could be that the control board is faulty. The control board sends power to the ignition system, which fires up the system. However, if the control board is defective, it may not be sending power to the ignition system, which will prevent the furnace from remaining on.

A defective pressure switch, which means it isn't closing when the burner has sufficient airflow, could also be causing the firing-up process to delay. If the ignition process delays, it will cause the furnace to shut off. If the flame sensor does not detect a flame, it will also cause the furnace to shut off.

Tools You'll Need for the Job

4-In-1 Screwdriver

Lennox furnace troubleshooting often involves inspecting the unit's inner components. Therefore, you will need a 4-in-1 screwdriver to remove the furnace covers and inside panels in the burner, so that you may access the parts.


Depending on where your furnace is located, it may be dark in and around your unit, in which case, you will need a flashlight for viewing the furnace area.


For more extensive Lennox furnace troubleshooting, such as checking the system's voltage, current, and resistance, you may consider purchasing a multimeter, which is an electronic measuring instrument with probes that allow you to connect to various furnace components.

Lennox Furnace Troubleshooting

Check the Thermostat

When Lennox furnace troubleshooting, first thing's first. If your furnace won't come on or is not putting out heat, simply check that the thermostat is on and set to the correct setting. When the thermostat setting is set too low, it will cause your furnace not to put out heat.

If your thermostat uses batteries, you should also check the batteries to see if they need to be replaced, which will also cause the system not to come on, and replace the batteries if needed to see if it helps.

Check the Power

A tripped breaker is another common cause of a furnace not coming on, so check the circuit breaker to be sure it is not tripped. If it is, simply flip the breaker back on to see if it helps.

You should also check that the furnace switch, which is usually located on the side of the furnace, is flipped to on, and then flip the breaker off and then on again to see if it helps.

Check the Air Filters

If your Lennox furnace uses a disposable filter, it may need to be changed. In fact, it is recommended that the filter is changed once every three months. If your filter hasn't been changed in a while, simply check the filter to see if it is dirty. Afterward, replace it with a clean filter to see if it helps.

Check the Pilot Light

If the pilot light is out, follow the instructions in your owner's manual on how to relight it, and then try to relight it. If it still does not come on, the pilot light housing might need to be cleaned. Simply turn off the gas leading to your furnace, and then clean the light housing.

If the pilot light still does not come on, there could be an issue with the gas flow at which time you will need to contact a professional HVAC technician for help.

Caution Notes

Because various furnace components involve dealing with gas and fire, which can be dangerous if not handled properly, if you are not comfortable with performing Lennox furnace troubleshooting on your own, it is best to contact a professional technician who is highly skilled and trained in dealing with furnaces for the safe inspection and repairs of your unit.

Likewise, there are some repairs that should be left strictly to the professionals, such as replacing the gas valve. This requires calibration to your home's furnace specs and gas pressure because it can damage the part as well as your system if it is not installed or calibrated properly.

Final Thoughts

Lennox furnace troubleshooting is simple and can be done on your own in just a few steps. In some cases, can even prevent the need to contact a professional. It also helps gives you a better understanding of your unit and how it works. Troubleshooting your Lennox furnace can be rewarding, knowing there are some things you can do on your own to help remedy the problem.

Wouldn't it be great if your Facebook and other friends knew they had the same ability? Feel free to share this article with them! In the meantime, after trying these Lennox furnace troubleshooting tips, be sure to share your insight with us.

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