Lennox Furnace Not Staying On

Lennox furnaces have a sticker on the front panel with the standard diagnostic codes for you to detect its current status. Use this guide or refer to the manual to decipher the meaning of the flashing, blinking, or other responses of the LED lights if your Lennox furnace is not staying on.

A Lennox furnace not staying on may be due to short cycling, a faulty thermostat, dirty flame sensor, malfunctioning gas valve or pressure switch, electrical connectivity issues or bad control board, low or blocked airflow, and frozen or clogged condensate line. 

A Lennox furnace not staying on can have one or more causal factors. A few problems are easier to remedy than others, but you have to detect the actual cause first. This article lays out all the common issues you are likely to encounter and the most appropriate solution for each.

Troubleshooting a Lennox Furnace Not Staying On

Here are the common reasons for a Lennox furnace not staying on:

  • Your thermostat instructs the furnace to shut down
  • A faulty or dirty flame sensor is stalling the ignition
  • The pressure switch fails to close or remains open
  • Malfunctioning gas valve is shutting the burner
  • Airflow into the furnace is inadequate for combustion 
  • The condensate line or drain is clogged or frozen
  • Bad circuit board or continuity issues
  • The furnace is short cycling due to one or more reasons   

Your first response to inspect any of these common problems depends on the precise scenario when your Lennox furnace stops working. Does it stay on for quite a while, warming your house, and then shuts down abruptly? In such cases, check the thermostat, return vents, and air filters.

If your Lennox furnace starts or ignites but shuts down soon after, inspect the flame sensor, gas valve, pressure switch, and circuit board. Also, you may have a short cycling issue if the Lennox furnace starts and stops frequently without any evident fluctuation in power and gas supply. A faulty control board or malfunctioning draft inducer motor may prevent your Lennox furnace from igniting, or it may not stay on. 

Some of these issues you can fix yourself while others require professional services. Let’s take a look at these issues and how to fix them.

Read: Lennox Furnace Problems

1. Your Thermostat Instructs the Furnace to Shut Down

A faulty thermostat sensor may misread the temperature in your house and thus erroneously shut down the Lennox furnace. Also, you may have frayed wiring and other connectivity issues affecting the thermostat, air handler unit, and furnace. Thus, conduct a thorough diagnosis, checking the thermostat settings, the sensor, and all connections.

In some cases, the thermostat in another room or zone may detect the preset temperature, but the space you’re in might not be desirably warm. Thus, it may not be the furnace’s fault. Review the thermostat installation and temperature settings, as these are likely to blame.  

Read: Why Lennox Furnace Is Not Staying ON

2. A Faulty or Dirty Flame Sensor Is Stalling the Ignition

A dirty or faulty flame sensor is the most common cause for a Lennox furnace not staying on. Go to the furnace and check the LED lights. Observe the flashing or blinking pattern and read the relevant code. Or, you can simply watch the furnace burner ignite and shut down in a few moments. A Lennox furnace usually retries to ignite the burner up to four times before stopping. 

The solution is straightforward. You need to clean the flame sensor. If it still doesn’t detect the flame and shuts off the gas supply, the only remedy is replacing it. 

Watch this video to clean a dirty Lennox furnace flame sensor:

Read: Why Lennox Furnace Not Igniting?

3. The Pressure Switch Fails to Close or Remains Open

The pressure switch in a Lennox furnace should close after you turn on the power, the fan starts running, and the gas flows into the unit. A faulty switch won’t close even after the requisite negative pressure has built up inside the metal housing, thus shutting off your Lennox furnace. 

Check the pressure line to see if it’s clogged. It’s possible you’ll have to replace the pressure switch. In some cases, the pressure switch and line may be fine, but the gas valve could be broken. Thus, you have to resort to an elimination process to detect the specific cause in your Lennox furnace. 

Here’s a video tutorial on changing a Lennox furnace pressure switch:

4. Malfunctioning Gas Valve Is Shutting the Burner 

Like the pressure switch, the gas valve in your Lennox furnace can shut down the unit abruptly if natural gas or propane isn’t permitted to flow in. The only way to diagnose whether or not a gas valve is broken is by measuring the input and output pressure. If there’s no output pressure, you’ll need to replace the gas valve 

Watch this video to measure the pressures at both terminals and change the gas valve:

Internal components, like the pressure switch or gas valve, will shut down a Lennox furnace soon after you turn on the power. Hence, you may have other problems if your unit isn’t staying on through the night or shuts down after a substantial period of functioning without glitches. 

Read: Lennox Furnace Error Codes – Troubleshooting Guide

5. Airflow Into the Furnace Is Inadequate for Combustion 

A Lennox furnace needs adequate airflow into the burner chamber for the unit to operate. The component responsible for this process is the return duct, with all its vents recirculating cold air from all around your house back into the furnace. Thus, check the return vents and air filters. 

Clogged filters and any obstruction blocking the airflow through the return vents will reduce the volume of combustible air inside the Lennox furnace. As a result, the unit’s temperature spikes, and the furnace will shut down per its safety feature to prevent a hazard. 

Clean all the air filters on the return vents. Remove any obstruction such as furniture from the front or around the vents. Also, inspect the return duct, if possible, to verify no surprises remain inside. Notably, a failing or broken blower fan is an evident problem detected instantly. 

Read: Best Furnace Flame Sensors

6. The Condensate Line or Drain Is Clogged or Frozen

Your Lennox furnace not staying on may be due to a clogged or frozen condensate line. Most furnaces can sense water buildup and lack of adequate condensate drainage. Yet, the furnace may continue to run normally until the problem is severe and beyond manageable for the unit. 

If your Lennox furnace shuts down abruptly in the middle of the night or without any of the other problems discussed in this guide, you may want to check the condensate line out of the unit and the drainpipe outside your house. If either is clogged, a cleaning should solve the problem. It’s also common for these lines to freeze in extreme winters, in which case, you’ll need to address that. 

This video isn’t of a Lennox furnace, but a clogged or frozen condensate line looks similar:

7. Bad Circuit Board or Continuity Issues

Testing for power connectivity is a complicated exercise, especially if you’re unfamiliar with the Lennox furnace circuit board, the various wires inside the unit, and the connections with the air handler and thermostat. Ideally, you should get an expert to test the voltage, amperage, and any connectivity issues for you.

8. The Furnace Is Short Cycling Due to One or More Reasons   

Almost all HVAC systems, including Lennox furnaces, are vulnerable to short cycling. A furnace too large for a house will shut down and restart frequently because of quick heating or the user’s selected temperature setting. Also, hardware components may cause short cycling as they fail.

Short cycling is a serious problem as it can impair the efficiency and, more importantly, the longevity of a Lennox furnace. Contact a certified HVAC technician and remedy the problem. It’s not safe to investigate a short cycling problem if you’re not trained in handling furnaces. 

Final Thoughts

A Lennox furnace not staying on could be due to a number of reasons. And some of these problems warrant a professional inspection. Use the troubleshooting steps in this guide to rule out or solve the common issues in Lennox furnaces.