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Lennox Furnace Not Staying On? Top 4 Causes (+ Fixes)

Got a Lennox furnace that shuts off almost as soon as it kicks on? It can be frustrating because you don’t get to enjoy the warmth you crave so much in winter. Not just that, a furnace that turns off quicker than programmed increases your energy bills and puts the furnace at risk of premature breakdown.

Your Lennox furnace isn’t staying on because of insufficient airflow, thermostat issues, a dirty flame sensor, or the furnace is too large. Fixes include cleaning clogged filters, opening heat registers, fixing the thermostat, cleaning the flame sensor, and buying a smaller Lennox furnace.

This article discusses the reasons why your Lennox furnace isn’t staying on in detail. I also explain how to fix each issue so that your furnace stays on.

Insufficient Airflow

If your Lennox furnace isn’t staying on or shuts off every time it comes on, it could be due to a lack of airflow. 

The following problems are the most common causes behind low airflow:

Clogged Filters

If you haven’t changed your furnace filter for some time, it may have become clogged with dirt and won’t pass out hot air as required. Hot air builds up in the heat exchanger and leads to overheating issues.

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 for safety reasons. This is a telltale signs that it’s time to clean the filters. 

How To Fix

  1. Turn off the furnace for safety.
  2. Remove the dirty air filter and slide in a new one. 

Note that 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, large box, or curtain in front of your heat registers and grilles, warm air can’t find a way to exit the system. Instead, the air accumulates inside the furnace, causing the unit to shut down as a safety measure. 

How To Fix

  1. Keep the heat registers open.
  2. Remove any objects that may be blocking the airways.
  3. Check to see that 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 requires more energy, you’ll 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 dirt and debris 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.
  3. If the blades and other components are worn out, you must replace the entire blower motor. A blower motor replacement should be 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 still won’t stay on, it’s time to check the thermostat. 

The thermostat tells your heating system what to do. If it’s broken or malfunctioning, it may send incorrect signals — your furnace will keep turning off and on repeatedly. 

Things like dead batteries or old wiring can make a thermostat malfunction. 

The placement of your thermostat matters, too; if it’s 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 they’re dead and replace them accordingly.
  2. Ensure the wiring is done correctly by referring to the owner’s manual.
  3. For advanced fixes that require replacing old wiring or moving the thermostat to a different location, call a professional. 

Dirty Flame Sensor

The flame sensor is one of the safety features installed in a heating system to ensure that gas is burning, not escaping through the vents. 

A steady flame will burn in the burner box if your furnace is working correctly. If the flame sensor checks the burner assembly and doesn’t sense any flame, it’ll 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’ll close the gas valve and, without gas, your furnace can’t stay on. It’ll cycle on and off repeatedly because the sensor keeps 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. 
  3. Remove the sensor.
  4. Using steel wool or emery cloth, clean the sensor to remove any dust or corrosion. 
  5. Wipe off any dust from the pilot and igniter.
  6. Put the sensor back in place, close the combustion chamber’s door, and turn the furnace back on. 

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

Oversized Furnace

Your furnace has to be sized right to function correctly. If it’s too large for your house, it’ll heat the rooms too quickly and 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’s not the right size for your home. 

How To Fix

Unfortunately, the only way to solve this problem is to purchase a new, properly-sized furnace. 

Consult an HVAC expert to see if you have the correct furnace size for your home. They can also tell you what furnace will work best for your home.

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 that: 

  • You have sufficient airflow.
  • The thermostat is working correctly.
  • The flame sensor is clean.
  • The furnace is the right size.

While you can implement most of the solutions listed here yourself, call in an expert if you can’t figure out or solve the problem yourself.


  • 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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