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Lennox Furnace Not Heating? Top 6 Causes (+ Fixes)

When the cold season sets in, you want your Lennox furnace to be working optimally. A heating system that won’t produce warm air when needed can be frustrating, as it means putting on thick clothes and enduring long hours of discomfort until the repairman shows up. But what caused the Lennox furnace to stop heating and is there a way to fix it?

The most common reasons for a Lennox furnace not heating include problems with the igniter, flame sensor, flame rollout switch, and wrong thermostat settings. Fixes include cleaning or replacing the igniter and flame sensor, checking the rollout switch, and changing the settings on your thermostat.

This article will set you up with several DIY solutions for a Lennox furnace that isn’t heating up.

Cracked or Faulty Igniter

If your furnace comes on but doesn’t produce any heat, the ignition system could be the cause. Usually, when your thermostat kicks on to start the furnace, the igniter heats and glows to light the gas burning unit. 

If the igniter is cracked or faulty, combustion can’t start and your furnace won’t produce any heat.  

How To Fix

  1. Turn off the furnace’s power supply.
  2. Remove the igniter from the unit and check if there are cracks on it. If it’s cracked, get a new one.
  3. If there are no cracks on the igniter, test it for continuity using a multimeter. If no continuity is displayed, replace the igniter. 
  4. If you’re using one of these older models that use a pilot light, check whether the light is still on. If the pilot is out, use the equipment owner’s manual guidelines to relight it. 

Dirty or Defective Flame Sensor

As the name suggests, a flame sensor senses (detects) the presence of a flame inside the furnace burner. A furnace will heat only if a flame is seen inside the burner assembly. If no flame is registered, the sensor will shut the furnace off immediately to prevent potential carbon monoxide leaks into the home. 

If the flame sensor is dirty or not working right, it may not detect the flame. Hence, the control board won’t send any voltage to the gas valve, and the furnace won’t heat. 

How To Fix

  1. Turn off the furnace and close the gas valve. 
  2. Unscrew the flame sensor and pull it out gently.
  3. Visually examine the sensor. If the insulation is intact, but there is soot or corrosion on it, the sensor may just need some cleaning. Grab an emery cloth and clean away the soot and corrosion.
  4. If the insulation is cracked, replace the sensor.
  5. Turn the power on to check if your furnace heats. 

Watch this video to have a better understanding of how to clean a dirty Lennox flame sensor:

Tripped or Defective Flame Rollout Switch

The furnace rollout switch is located near the gas burner box. Its job is to monitor the heat around the burners. If there’s excess heat in the unit, the switch trips to stop the ignition process. The next time you turn on the furnace, it won’t heat up.

How To Fix

If your Lennox furnace isn’t heating, check to see if the rollout switch is tripped or just faulty. A tripped switch could mean that the flames are burning back in the wrong places. 

Tripping could be caused by a clogged vent, cracked heat exchanger, or low gas pressure.

Here’s how:

  1. Locate the rollout switch and check to see if it has tripped. While you can fix the problem by simply returning the switch to the on position, a tripped flame rollout switch could be an indication of a bigger issue in the burner box. You’ll be better off working with an HVAC expert.
  2. If the switch isn’t tripped, test it for continuity using a multimeter. A rollout switch that doesn’t have continuity is defective and should be replaced. 

Dirty Furnace Filter

Contrary to what many people think, a furnace filter’s primary job is not to keep the air clean — it’s to protect the furnace from getting damaged. 

Furnace filters are designed to trap dirt, debris, dust, and other objects that enter our homes through the indoor air supply. Improving indoor air quality is just a nice bonus!

If your furnace filters are clogged with elements, it can be difficult for hot air to effectively pass through the unit to warm your living area. So, if your Lennox furnace is turned on and no heat seems to come out, dirty filters could be restricting airflow.

How To Fix

  1. Locate your furnace filter. Air filters are usually found in the system’s return air vent. 
  2. Open the return and remove the old filter. 
  3. Check its size to make sure you have the right filter for replacement. 
  4. Following the arrows printed on the new filter, gently slide the filter into the return. 
  5. The standard disposable furnace filters should be replaced every 90 days. If you have pets or someone who suffers from allergies in your home, you may want to replace your filters every 30 days. 

Changing a Lennox furnace filter is a pretty straightforward process. Here is a video to make the process even simpler for you:

Wrong Thermostat Settings

When your thermostat has the wrong settings, it might seem like the furnace comes on, but no heat is produced through the vents. The thermostat must first activate a heating cycle for hot air to come from your heating system. 

As simple as this may sound, setting your thermostat right could be all you have to do to get your Lennox furnace heating again.

How To Fix

  1. Check to see the thermostat is set to HEAT and to a temperature higher than the current room temperature to trigger the system to produce heat.
  2. Make sure it’s getting power and that the interior components are clean. This ensures the thermostat is sending the right signals to the furnace. 
  3. Check to see whether the fan is set to ON or AUTO. When the fan is in the ON position, it continually blows air through the vents, and you may feel cold air coming out. 
  4. Set the fan to AUTO position. That way, it’ll only turn on when the air heats up to the temperature marked on the thermostat. 

Closed Heat Registers

It may sound unbelievable, but sometimes the only “troubleshooting” your Lennox furnace needs to heat is opening the heat registers. Shut heat registers are nothing new to families with young kids, especially if the registers are located at places where children can tamper with them. 

However, if too many are closed, heat can build up in the furnace, causing the system to shut down. Once the system is shut off, no heat can come out through the vents. Blocked heat registers also cause the furnace to work harder than designed, shortening its lifespan. 

How To Fix

  1. Check to see the heat registers are open. 
  2. Move any objects that may be blocking the registers and preventing them from distributing hot air properly.

Final Thoughts

If your Lennox furnace kicks on, but no heat comes out through the vents, and none of these fixes seems to solve the problem, hire a professional.


  • 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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