Lennox Furnace Not Staying On? Common Causes and Fixes

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Got a Lennox furnace that stops almost as soon as it kicks on? It can be frustrating because you certainly don’t get to enjoy the warmth you crave so much when it starts to get cold. Not just that, a furnace that turns off quicker (and more frequently) than programmed ridiculously increases your energy bills and puts the equipment at risk of premature breakdown.

Several factors can contribute to a Lennox furnace not staying on or turning off and on repeatedly. The most common include insufficient airflow (clogged filters, blocked heat registers, malfunctioning blower wheel), thermostat issues, a dirty flame sensor, and wrong furnace sizing.  

Here, I have discussed these issues in detail and suggested some solutions that you could implement to keep your furnace on. But if you would like to skip the hassle of doing the work yourself and have an HVAC professional examine your equipment, I have provided a form at the bottom that will hook you up with a certified HVAC technician near you. Keep reading!

Insufficient Airflow

If your Lennox furnace is not staying on or shuts off every time it comes on, it could be that the airflow is too low. A few things can cause low airflow like:

Clogged Filters

If you haven’t changed your furnace filter for some time, it will become clogged with dirt and won’t pass out hot air as required. This air will build up in the heat exchanger and will eventually cause it to overheat.

Now, your furnace comes with a safety mechanism that shuts the equipment off when it gets too hot so, when the heat exchanger overheats, the system automatically shuts off to stay safe. Well, this is a good thing, but it’s certainly not what you want when it’s freezing outside. 

How To Fix

  1. Turn off the furnace for safety.
  2. Remove the dirty air filter and slide in a new one. You must replace your furnace filter every three months. If you have pets, allergies, or a respiratory problem, you may want to change the filters more frequently, say, every month. 

Below is a short video to help you better understand the process of replacing a Lennox furnace filter:

Closed Heat Registers

Blocked air supply vents could be another reason why your furnace keeps shutting off. If you have a chair, a box, or even a curtain in front of your heat registers and grilles, it could be difficult for warm air to exit the system. Then that air accumulates inside the furnace, causing the unit to shut down as a safety measure. 

How To Fix

  1. Keep the heat registers open; remove any objects that may be blocking the airways. 
  2. Check to see the dampers along the ductwork are appropriately positioned. 

Malfunctioning Blower Wheel

If the blower motor isn’t circulating air correctly, your furnace will overheat, causing the limit switch to turn off the heating system to prevent damage. A blower wheel that is beginning to fail will usually make uncharacteristic sounds as it struggles to remove the hot air out of the unit. 

And since this will require more energy, you will also notice a significant spike in your utility bills. 

Blower wheel malfunctioning is usually caused by dirt buildup on the blades. Constant care and maintenance, including replacing your filters regularly, can help prevent elements accumulation on the blower blades. 

How To Fix

  1. Turn off the furnace’s power supply and remove the blower motor.
  2. Thoroughly blow the dirt off the blades with a vacuum cleaner or scrape it off with a small brush. Do this gently; you don’t want to stress the wiring or other components on the blower.
  3. If the blades and other components are worn out, you must replace the entire blower motor. Blower motor replacement is almost always left to a professional. 

For more insights on how to clean a furnace blower wheel, check this video out:

Thermostat Issues

If air is flowing properly, but your Lennox furnace will still not stay on, it’s time to check the thermostat. The thermostat tells your heating system what to do, and if it is broken or malfunctioning, it is likely to send incorrect signals, and your furnace may start then stop, or keep turning off and on repeatedly. 

Things like dead batteries or old wiring could make a thermostat not function efficiently. Where you place your thermostat matters, too; if it is installed near a heat register, direct sunlight, etc., the sensor will think your home is warm enough and will shut down the furnace. 

How To Fix

  1. If your thermostat uses batteries, check to see if the batteries are dead and replace them.
  2. For advanced fixes like replacing old wiring or moving the thermostat to a different location, it’s best to involve a professional. 

Dirty Flame Sensor

The flame sensor is one of the safety features installed in a heating system to ensure the gas is actually burning and not escaping the unit through the vents. If your furnace is working correctly, there will be a flame burning inside the burner box. If the flame sensor checks the burner assembly and doesn’t find any flame, it will turn off the furnace to prevent unburned gas from exiting the unit. 

When the sensor is dirty or corroded, it can’t read the flame accurately or won’t read the flame at all. So, it will close the gas valve, and without gas, your furnace can’t stay on. It will cycle on and off repeatedly because the sensor is sending incorrect signals. 

How To Fix

  1. Turn off power to your furnace.
  2. Open the door of the combustion chamber to access the flame sensor. Remove the sensor.
  3. Using steel wool or emery cloth, clean the sensor to remove any dust or corrosion. 
  4. Wipe off any dust from the pilot and igniter.
  5. Put the sensor back in place, close the combustion chamber’s door, and turn the furnace back on. 

Watch the video below to understand how to clean a Lennox furnace flame sensor:

Oversized Furnace

Your furnace has to be sized right for it to function correctly. If it is too large for your house, it will heat the rooms too quickly and then shut down. However, without the furnace working consistently, your home will soon cool off, then the process will start again. 

If your Lennox furnace won’t stay on, there is a chance it is not the right size for your home. Contact a professional to help you figure out if wrong sizing is the cause of the problem. 

How To Fix

  1. Unfortunately, the only way to solve this problem is to purchase a new, properly-sized furnace. Consulting an HVAC expert can help you get the correct furnace size for your home. 
  2. However, if the furnace is brand new and the installer gave you a labor warranty, simply inform them about the problem to resolve it. 


If your Lennox furnace isn’t staying on, check to see you have sufficient airflow, the thermostat is working correctly, the flame sensor is clean, and the furnace itself is sized right. While you can implement most of the solutions listed here yourself, it would be wise to call in an expert if your furnace continues to act up. 

The form below will help you find and contact an HVAC professional near you. With an expert, you will not only have the issue fixed fast but also get foolproof maintenance tips that will keep your system working optimally all season. 

Steve Rajeckas

Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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