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.
Among the many emails we received from homeowners who use Lennox heating systems that fail to turn on, the common complaint was that the thermostat would read “heat on,” but the furnace blower never kicked on, and the ignitor never started (even when the AC and fan both work fine)
If this is similar to your situation, there are a number of factors that could be the culprit, but most of the troubleshooting for this problem can be done completely DIY, so you’ll likely not end up spending money on a service call.
Troubleshooting Lennox Furnace Not Turning On — Step-by-step Guide
Remember that the first action is to observe the control board for any of the furnace error codes (often in green or red color) to help diagnose the problem.
If there are no furnace error codes, these quick fixes could be just what’s needed to get your furnace up and running again. So, let’s get started.
| Possible Causes | Solution |
| Improper thermostat settings. | Ensure the thermostat is set to the desired temperature. |
| Problem with the power supply | Confirm power is reaching the furnace; check the fuse. |
| Tripped bakers or defective fuses | Reset tripped breakers; replace blown fuses. |
| Malfunctioning Pilot Light | Relight the pilot light if it is out; check for issues. |
| Blocked or faulty air filters | Clean or replace dirty filters to ensure proper airflow. |
| Fault from limit switch | Test and reset the limit switch if necessary. |
| Defective blower motor | Check for motor issues, lubricate if needed, or replace. |
| Blocked condensate drain | Clear any blockages in the condensate drain. |
Read on to find out how each of these issues could be the culprit behind your furnace not turning on and get more precise guidance on getting the system to work correctly again.
Checking the Thermostat 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
- Check to see if your thermostat is set to ‘heat.’
- Raise the thermostat temperature about 10° F higher than what is currently indicated on the device to see if the furnace turns on.
- If this doesn’t help, check the thermostat’s power supply. If your thermostat is battery-operated, try replacing the batteries.
- If getting new batteries doesn’t work, check the circuit breaker to see if the thermostat circuit has tripped. A tripped circuit could signal a power surge; try to find out what’s causing the overflow.
- To find out if the thermostat is faulty, test it for continuity with a multimeter. If no continuity is displayed, replace your thermostat.
Verifying the 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
- Turn the switch back on and wait for your system to respond. Some models may take several minutes to start up.
- 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.
- 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.
If you need further directions on identifying why your Lennox furnace isn’t getting power, we have a separate guide to put you through.
Inspecting the Circuit Breakers and Fuses
The furnace relies on electrical power to function, and disruptions in the power supply can lead to malfunctions.
If the fuse is blown, it interrupts the electrical flow to the furnace and prevents it from operating. Regular wear and tear, electrical surges, or faulty components can cause circuit breakers to trip or fuses to blow.
How To Fix
- Start by checking the circuit breaker panel.
- Furnaces typically have dedicated circuits, so look for a breaker labeled ‘furnace’ or ‘HVAC.’
- Ensure it’s in the ON position
- If it’s tripped, reset it and try turning on the furnace again.
- Inspect the fuses in the furnace itself or the main electrical panel
- Replace any blown fuse with ones of the same amperage.
Examining the Pilot Light
The pilot light is a small flame that serves as an igniter for the gas in your Lennox furnace, initiating the heating process. If the pilot light is extinguished or not functioning correctly, the furnace may not start.
Several issues could lead to a pilot light going out, such as a draft, a faulty thermocouple, or a clogged pilot orifice.
How To Fix
- Locate the pilot light access panel on your furnace. (near the bottom of the unit)
- Open the panel and look for the pilot light. If it’s not lit, try relighting it. Lennox brand explains how you can properly light a Pilot in the video below: