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Lennox Furnace Not Getting Power? Top 4 Causes (+ Fixes)

After months of inactivity, it’s common for home heating systems to malfunction. If your Lennox furnace isn’t getting power all of a sudden, don’t panic. It’s probably something as mundane as a tripped circuit breaker that you can fix yourself. 

Your Lennox furnace isn’t getting power because of an incorrectly set thermostat, dirty air filters, tripped breakers, or a lack of gas. Easy ways to fix this include checking the thermostat, cleaning the air filters, inspecting the breaker box, and ensuring the gas valve is open.

This article will explain why your Lennox furnace isn’t getting power and a few ways to try to fix that.

Incorrectly Set Thermostat

Your thermostat tells the furnace when to power on and off. So, if it’s not set correctly or if it’s malfunctioning, your Lennox furnace won’t get the right signal. 

How To Fix

If your Lennox furnace isn’t getting power, you need to check the thermostat settings. You may also want to check other thermostat components like batteries and wiring to ensure they are in good condition. Here’s how:

  1. Ensure the thermostat is set to HEAT mode and displays a temperature higher than the actual room temperature. 
  2. Check for dead batteries. Modern thermostats will have a Low Battery icon flashing on their control screen. In that case, replace the batteries. 
  3. Open the thermostat cover to see if there’s dirt on the inner circuitry. Dust and debris can mess with thermostat sensors, causing the device to send the wrong information to the furnace. 
  4. Remove any dirt and dust you find with a soft brush. 
  5. Check the wiring. If a wire is loose or damaged, consider hiring an HVAC expert; they can fix any wiring issues easily.  

Dirty Air Filters

A furnace air filter collects dirt, dust, and debris from the air passing through the vents, making it safer for the furnace to operate. The improved air quality is also nice. Of course, like any type of filter, it needs to be cleaned or replaced to function efficiently. 

If you don’t replace your furnace air filter regularly, dirt will accumulate on the system’s internal components, causing them to malfunction. Accordingly, if some of the parts malfunction, the furnace’s safety features will shut the unit down every time you turn it on to prevent damage. 

How To Fix

If the thermostat is working fine, but your Lennox furnace still isn’t getting any power, check the filter; maybe it’s clogged. While at it, check if dirt has accumulated on the:

  • Fan
  • Gas valve
  • Heat exchanger
  • Ignitor
  • Any other components

If you see any signs of dust and debris, clean them with a soft brush and a can of compressed air.

Since furnace repair projects are costly, change the filter regularly to keep the system’s components in good condition. 

The standard disposable furnace filters need to be replaced every 90 days. People with pets or respiratory allergies should replace theirs every 30 days.

Here’s how to replace a dirty air filter:

  1. Turn off the furnace’s power.
  2. Locate the air filter. It’s typically positioned near the intake/output blower.
  3. Remove and inspect it. If it’s hard to see through, your filter needs changing. 
  4. Write down the size of the old filter to ensure the new one will fit.
  5. Throw the dirty filter away and slide in the new one. Use the arrows indicating the direction of the airflow to install the filter. 
  6. Turn the system back on. 

If the furnace doesn’t power on, an internal component may be clogged. You should have the furnace checked by an HVAC technician.

Replacing a furnace air filter is one of the easiest things to do. Here’s a two-minute video that’ll guide you through the process in more detail:

Tripped Breakers

Even though Lennox furnaces are gas-fueled, some components still require electricity to function. The most crucial one is the blower, as that’s the part that circulates warm air throughout your living area. 

Most furnaces these days turn on with an electronic ignitor instead of a pilot light, and this ignition requires electricity. 

If a circuit breaker is tripped, your furnace may have trouble getting power. One of the reasons circuit breakers trip is an overworking blower. 

If something prevents sufficient airflow to the furnace, such as a dirty filter, the air blower must work harder than usual to compensate. The harder the blower works, the more energy it consumes. This can cause the circuit breaker to trip.

Other issues that may cause your furnace blower to overwork include duct leaks, home air leaks, and blocked air supply vents. But there are times when tripping is caused by a component that isn’t part of the furnace, especially if the furnace shares a circuit breaker with multiple electric gadgets. 

In some instances, tripping may be caused by a random power surge. Other times, the breaker itself may just be faulty. 

All in all, a tripped circuit breaker can cause a heating system not to power on. This is one of the things you should check if your Lennox furnace isn’t starting. 

How To Fix

  1. Turn off the furnace’s power.
  2. Check your furnace filter. If it’s dirty, install a new one. 
  3. Go to the circuit breaker box and carefully flip the tripped circuit back on. 
  4. Turn on the furnace to check if it works. 

If your furnace still isn’t getting any power, or if the breaker trips immediately after you turn it on, there could be a bigger problem with the system. Contact a professional so they can inspect it. 

Below is a video showing you how to check your HVAC circuit breakers:

The Gas Supply Is Shut Off

If your furnace isn’t getting power, it may be because no gas is being supplied to the system. Sometimes the gas valve may only be partially turned on, either due to safety concerns or through carelessness, preventing a sufficient gas supply to the heating system. 

When working with a gas-fueled Lennox furnace, you need to keep the valve turned all the way on to ensure the unit is getting enough gas. Otherwise, your furnace won’t power on. 

How To Fix

  1. Power off the furnace.
  2. Locate the gas control valve and check how it’s positioned. 
  3. If the gas is shut off, the valve handle will be turned perpendicular to the outlet line coming from the control point. 
  4. Turn the valve so that the handle is positioned parallel to the gas outlet. This will open the valve and ensure your furnace receives enough gas. 
  5. Turn the furnace’s power back on and see if the system works. If not, call in an expert. 


Furnace troubleshooting is usually best left to HVAC specialists. If you’re having trouble getting your Lennox furnace to start, I recommend involving a professional. 

Before you do, though, check to see you don’t have any thermostat issues, the air filter is clean, the circuit breakers aren’t tripped, and the gas control valve is open.


  • 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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