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:
- Check for any drafts in the area that might be blowing out the frame.
- Clean the pilot orifice using a small brush to remove any debris that could be blocking the gas flow.
- Ensure that the thermocouple, a safety device that senses the pilot flame as the igniter, is positioned correctly and not damaged. If damaged, you need the attention of an HVAC technician for replacement.
Cleaning or Replacing the Air Filters
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 to whistling sounds.
How To Fix
- Turn off the furnace.
- Open the filter compartment door to expose the old filter.
- Pull the filter out and examine how dirty it is. If it’s really dirty, replace it.
- Install the new one and 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.
- Replace the filter cover and switch the furnace power back on.
- Replacing a furnace filter is a simple process, but it can be 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:
Testing the Limit Switch
The limit switch in a Lennox furnace is a safety device that monitors the temperature inside the furnace and prevents it from overheating. The device has a set temperature threshold and will shut off the burner when it detects unsafe temperature to prevent damage or a potential safety hazard.
Over time, the switch may wear out, accumulate dust, or malfunction, causing it to falsely signal to overheat and preventing the furnace from igniting.
How To Fix
- Turn off the power to the furnace
- Locate the limit switch near the burner assembly or on the furnace wall
- Use a multimeter to check for continuity across the switch terminals
- If there’s no continuity when the furnace is excellent, or if the switch is visibly damaged or covered in debris, it may be faulty and require replacement.
Inspecting the Ignition System
The ignition sensor or flame 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
- Turn off the furnace’s power supply.
- Remove the front cover to locate the ignition sensor. Your sensor will be located near the burners.
- If the sensor has rust or carbon deposits around it, it must be cleaned.
- 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.
- Once clean, clip the sensor back in and reattach the screw. Turn the furnace back on.
To further understand how to clean a Lennox flame sensor, check out this video:
Assessing the Blower Motor
Heated air goes throughout the home’s venting with the aid of the blower wheel, which is powered by the blower motor. So when your heating system fails to turn on or work, it could be traced to the blower wheel not moving freely due to a defective motor.
How To Fix
- Use a multimeter to determine if power is reaching the motor
- If the component is receiving voltage but doesn’t run, it is likely defective and needs replacement for your furnace to work.
- Use your hand to turn the blower wheel.
- If the blower wheel does not turn freely, it’s likely the blower motor bearings have seized.
- This also means the motor needs replacement.
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
- Turn off your furnace’s power supply.
- 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.
- Use a wet/dry vacuum to get rid of this water.
- Insert a plastic tubing or plumber’s snake into the drain line (the PVC tubing connected to the pan) to clear the clog.
- 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.
- Turn the power back on.
For more insights on how to clean a furnace condensate drain, watch this video:
Calling for Professional Assistance
If the issue persists after trying all of the above fixes, it may be a sign of a more significant electrical problem. It’s advisable to seek professional assistance.
Some issues are simply beyond DIY for safety reasons and to prevent further damage to your heating systems.
For example, when you eventually call in an HVAC technician, they could diagnose problems such as a faulty furnace main control board, defective draft inducer motor assembly, or even damaged blower assembly — all of which require the intervention of a professional.
Feel free to share in the comment section below which of these solutions worked for you.
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.