Why Lennox Furnace Not Igniting?

What can be more annoying than a furnace that fails to ignite?

Your Lennox unit might not be igniting because there is no gas supply, the air filter is dirty, and the ignition source has gone bad. An open high-limit switch, a faulty thermostat, and draft motor issues might also be to blame.

Why Is My Furnace Turning on But Not Igniting?

Your furnace might not be igniting for quite a few different reasons. But before moving any further, make sure that the power switch on the furnace and the gas valve in the gas line are in the ‘on’ position and the thermostat is switched to ‘heat’.

If the simple steps mentioned above didn’t fix the problem, then keep on reading.

Gas Supply

Why Lennox furnace not igniting? The most obvious reason might be that there is some kind of a problem with the gas supply.

If you’re on a city gas line, then ensure that everything is ok from their side. If you have a separate storage tank for liquid propane, then find the gas pressure gauge.

The gauge should show 10% or above. If not – you might have to invite the gas company for a fill-up.

Air Filters

If there is a problem with the airflow within the system, the furnace won’t be able to create the proper gas/air mixture. In such a case, your Lennox furnace won’t ignite.

The main thing that can restrict the airflow is a dirty air filter. Replace the filter and see if this helps.

Dirty hvac filters

Read: Why Does My Furnace Filter Get Dirty So Quickly

Ignition Source

  • Pilot light

Some units still have a pilot light. This little flame should be constantly burning inside the furnace.

Look under the unit to find out if the flame is lit. If it’s not, then try carefully lighting it up with a long match.

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  • Electronic furnace ignition

Igniters can be found under the unit as well. 

You would have to make sure that the component glows when you try to switch the furnace on. You should also hear a ‘click’.

However, repeated clicking and no result means that the system had failed to come on.

You can try gently cleaning the igniter to fix the issue. But don’t forget to turn the unit and the gas supply off before doing that.


If the thermostat is functioning correctly, then you are going to hear a few sounds when you set the thermostat to a higher temperature.

You will hear the fan, the swoosh sound of the ignition process, and, a few seconds after, the sound of hot air flowing through the ducts

If the system remains silent, then there is a problem with the thermostat.

Read: Why Does Home Thermostat Say HEAT ON But There Is No Heat?

An Open High-Limit Switch

A limit switch is a safety device that will shut the furnace down if there is a chance that the unit is going to overheat.

The first thing that you should do is inspect the air filter and replace it if it’s clogged or dirty.

If that didn’t help, then the best decision is to call an HVAC expert, as various serious issues can cause the high-limit switch to trip.

Read: HVAC UV Light: Everything You Need To Know

Draft Motor Issues

The main job of the draft motor (the blower) is to pull in the air and push it out of the house through a vent or chimney.

If the blower isn’t functioning, then the furnace will not ignite. You should hear humming noises coming from the system right before the burners ignite, if you don’t hear the sound, then the draft motor is dead.

If you hear the humming sound, but your Lennox furnace still doesn’t ignite, then there might be a blockage in the vent. Make sure that it is clear of snow, ice, debris, bird nests, and so on.

Does Lennox Furnace Have a Pilot Light?

Most of the newer models use spark ignition. However, if you have an old Lennox furnace, then it might have a pilot light.

It is extremely simple to figure out whether your unit has a pilot light or not.

Such furnaces have a small round knob on the gas valve that says ‘off’, ‘on’, and ‘pilot’.

Lennox Pilot Light Location

Open the burner assembly and look toward the bottom. You will see a knob that says off/on/pilot.

There, you’ll find three tubes that are attached to the base of the gas valve housing. One of them travels to the gas burners, while the other two (they are typically smaller) will lead you to the pilot light.

Of course, it is relatively easy to spot a small blue flame, if the pilot light is burning. But if it’s not – use a flashlight to locate the end of the pilot tube (that’s where the pilot orifice is).

How Do You Reset the Pilot Light on a Lennox Furnace?

Always read the instructions that you’ll find on the blower access panel.

  • Set the thermostat to the lowest possible setting.
  • Turn off the furnace and remove the burner access panel.
  • Locate the gas valve knob, push it in, and turn clockwise to ‘off’.
  • Wait for 5 minutes, so that the system has enough time to get rid of any leftover gas.

Warning! If you smell gas after this step, immediately call your gas supplier.

  • Behind the gas valve, you’ll see the pilot access panel – remove it.
  • Follow the tube from the gas valve to locate the pilot.
  • Turn the gas valve knob counterclockwise to ‘pilot’.
  • Push in the gas valve knob and hold it while you try lighting the pilot with a long match. 
  • Continue holding the knob for around 1 minute, after you have lit the pilot.

Tip: did the light go out? Repeat steps 3-9.

Warning! If the knob didn’t pop up once you released it or the pilot won’t stay lit even after a few tries, then call your gas supplier.

  • Put the pilot access panel back in place, turn the knob counterclockwise to ‘on’, replace the burner access panel, and turn the unit on. Don’t forget to change the thermostat settings.

How Do I Know If My Furnace Spark Ignitor Is Bad?

If your Lennox furnace doesn’t have a pilot light, then it uses spark ignition.

Here are a few signs of a faulty spark ignitor.

The furnace stops operating

If the unit won’t turn on at all, then the chances are high that there is a problem with the ignition. 

The furnace blows cool air

If your thermostat is set to heat, but the unit is blowing cold air, then it looks like the ignitor is not lighting the burner.

The furnace is short cycling

Short cycling is when your unit is turning on and off repeatedly. It is a safety feature that indicates that something is wrong.

The ignitor is not glowing

An ignitor is located in the same place where you would usually look for a pilot light. This element should get extremely hot, in order to ignite the gas/air mixture.

However, if you have turned the furnace on, but the ignitor is not glowing, then there is something wrong with the component. 

Read: Why Furnace Is Running But Not Blowing Air?

Lennox Furnace Ignitor Cleaning

In most cases, the ignitor will continue working even if it is dirty. But the element won’t get as hot, in case it’s covered with carbon deposits.

You can try cleaning the ignitor with compressed air. You can find such cans in stores and online; they usually come with long ‘straws’ which make it easy to apply the air directly to the ignitor.

You can also try manually cleaning your Lennox furnace ignitor.

Simply pull off the wires, loosen the screw, and gently pull the ignitor out. Carefully rub the ignitor probe with an emery cloth or fine steel wool.

Tip: don’t touch the ignitor with bare hands as the oil from your fingers might damage the element. 

Read: American Standard Furnace – Troubleshooting Guide