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Lennox Furnace Not Turning On? Top 5 Causes (+ Easy Fixes)

Lennox furnaces have been a vital winter appliance for many years, providing homeowners with reliable and convenient heating systems. But as with any home heating system, they too experience breakdowns, requiring constant care and maintenance. A common problem is a failure to come on, which could cause your house to get not only cold but also potentially freeze your water pipes, resulting in costly repairs.

If your Lennox furnace doesn’t turn on, check to see the thermostat settings aren’t changed, the filter isn’t clogged with dirt, the condensate drain isn’t blocked, and the ignition sensor isn’t dirty. Ensure the power switch is turned on, too, as the furnace might not be getting enough power.

Working with an HVAC technician can help you quickly address the problem, but before involving one, consider giving the following troubleshooting steps a shot. These quick fixes could be just what’s needed to get your furnace up and running again. So, let’s get started!

Check the Thermostat’s Settings

The thermostat is the central control point of your heating system; it regulates how much heat the furnace gives out and when. When you set your thermostat to a given temperature, the furnace will always work to keep your room at this desired temperature. If the room’s warmth starts to drop, the thermostat turns the furnace on to raise the temperature to the set point.

A wrongly set thermostat could be the reason for a Lennox furnace not turning on. Dead batteries could also be the problem, or the thermostat may be faulty.

How To Fix

  1. Check to see if your thermostat is set to ‘heat.’
  2. Raise the thermostat temperature about 10° F higher than what is currently indicated on the device to see if the furnace turns on.
  3. If this doesn’t help, check the thermostat’s power supply. If your thermostat is battery operated, try replacing the batteries.
  4. If getting new batteries doesn’t work, check the circuit breaker to see if the thermostat circuit has tripped. A tripped circuit could sign a power surge; try to find out what’s causing the surge.
  5. To find out if the thermostat is faulty, test it for continuity with a multimeter. If no continuity is displayed, replace your thermostat.

Check if the Filter Is Dirty

The furnace filter’s work keeps dust, hair, and other dirt particles that the return duct pulls in from reaching the blower fan. It also prevents contaminants from being recirculated, which helps keep your inside air clean.

If you haven’t replaced or cleaned your furnace filter for a while, dust and other objects may be clogging it up, preventing air from passing through. One way to determine whether a dirty filter is why your Lennox furnace isn’t turning on is by listening for whistling sounds.

How To Fix

  1. Turn off the furnace.
  2. Open the filter compartment door to expose the old filter.
  3. Pull the filter out and examine how dirty it is. If it’s really dirty, replace it.
  4. Install the new one and, if you can, mark the date you installed it so you can know when to do your next replacement. Experts recommend changing furnace filters every 3 to 6 months.
  5. Replace the filter cover and switch the furnace power back on.

While replacing a furnace filter is a simple process, it can be a little overwhelming if you attempt it for the first time. Below is a video that demonstrates how to replace a Lennox furnace filter to make the process less intimidating. Check it out:

Check if the Condensate Drain Is Blocked

The condensate drain is located underneath the evaporator coil, and its job is to channel out moisture from the air as the air passes through the coil. Generally, the coil sucks out humidity from the air, converts it into the water, and deposits it into the condensate pan. This water then exits the system through the draining line.

Over time, your condensate drain could get clogged with dirt, microbes, and sticky substances, preventing proper water flow out of the pan, which could cause flooding of the unit. This could result in the corrosion of metal parts inside the furnace, which could hinder the proper functioning of the equipment. If your Lennox furnace won’t turn on and no water is dripping from the exit pipe, chances are your condensate drain is clogged, and you need to act immediately.

How To Fix

  1. Turn off your furnace’s power supply.
  2. Locate your condensate pan and check if there is stagnant water in it. Standing water could be a sign that the drain line is blocked.
  3. Use a wet/dry vacuum to get rid of this water.
  4. Insert a plastic tubing or plumber’s snake into the drain line (the PVC tubing connected to the pan) to clear the clog.
  5. Find the line outlet, which will probably be located outside your house, and flush out obstructions with white vinegar or hot water and mild detergent.
  6. Turn the power back on.

For more insights on how to clean a furnace condensate drain, watch this video:

Check for Dirty Ignition Sensor

The ignition sensor is installed in a furnace to detect whether the system is burning gas. It uses the increasing heat in the furnace to recognize flames. If no flame is detected, it shuts down the furnace to prevent the unburned gas from escaping.

Dirt buildup can keep the ignition sensor from detecting the flame, and your Lennox furnace will not turn on.

How To Fix

  1. Turn off the furnace’s power supply.
  2. Remove the front cover to locate the ignition sensor. Your sensor will be located near the burners.
  3. If the sensor has rust or carbon deposits around it, it must be cleaned.
  4. Unbolt the dirty sensor from its current position and clean the metal rod with a piece of steel wool. Rub gently to avoid detaching the rod from its base.
  5. Once clean, clip the sensor back in and reattach the screw. Turn the furnace back on.

To further understand how to clean a Lennox ignition sensor, check out this video:

Check the Furnace’s Power Supply

Sometimes, a furnace won’t kick on because the power switch is turned off. While this switch will likely be located on the nearby wall, it’s easy for someone unfamiliar with your home to accidentally turn it off, thinking it’s a light switch.

How To Fix

  1. Turn the switch back on and wait for your system to respond. Some models may take several minutes to start up.
  2. If this doesn’t work, check the circuit breaker to see if the furnace circuit has tripped. If it has tripped, switch it back on.
  3. Check if you have blown fuses, too, as this could mean you had a power surge. Try to identify what caused the surge and address it.

Final Thoughts

A furnace that won’t turn on is the last thing you want on a cold winter night. The problem could be a result of wrong thermostat settings, a clogged filter, a blocked condensate drain, a dirty ignition sensor or the unit doesn’t have any power.

If you tried the solutions provided above and your Lennox furnace is still not turning on, or if it’s your first time attempting to fix a problem on a home heating system, it advisable to call a professional.


  • Jake Alexander

    Jake is a freelance writer from Pennsylvania who enjoys writing about science and sports. When he's not writing for Temperature Master, he can be found watching the NFL or playing basketball with his friends.