Lennox furnace troubleshooting

HVAC Logo IconBy Tom MoorAugust 1, 2023

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Lennox furnace not working? You may be able to solve the problem on your own with some Lennox furnace troubleshooting. Before you call a repair person, give these tips a try.

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Lennox furnace is not heating or not blowing hot air

First, check your power. Ensure no switches have tripped or fuses blown. If they have, fix them.

Next, check your thermostat. If it takes batteries, make sure they’re fresh. Your system should be on “heat” and “auto.” Choosing on “on” instead of “auto,” will make the furnace constantly blow air whether or not it’s actively heating.

Check the temperature setting. If someone’s lowered it, the heat won’t start until the home temperature dips below that setting.

Next, look at your air filter. If it’s full of dust, change it. You should change your Lennox furnace filter regularly. You can pick one up inexpensively on Amazon or at a store like Target. Our guide demonstrates how to change your furnace filter

Finally, clean the inside of your furnace. Turn off the power to the unit and open the cover. Gently dust the components inside with a soft cloth or canned air

If these Lennox furnace troubleshooting steps don’t work, call a technician. You may have a problem with your gas valve or blower motor panel. These aren’t DIY fixes. 

man fixing Lennox furnace

Lennox furnace is continuously running

If your furnace constantly runs, you likely need a new filter. See the above question for information on changing your furnace filter.

If that doesn’t help, check your thermostat. If you set it to “on” instead of “auto,” the fan will run constantly. Change it to “auto” and see if that resolves the issue.

Next, make sure there are no air leaks in your house. Energy.gov has good tips on how to do this. If you find any, repair with caulk or weatherstripping

Finally, ensure your ductwork is free of obstructions and leaks. You can see into your ducts by opening your registers and using a flashlight. Or you can hire a professional to inspect and clean your ducts.

If you still can’t get your Lennox furnace to stop running, call an HVAC pro. You may have a faulty limit switch.

Lennox furnace is starting and stopping

This pattern is commonly known as “short cycling.” If your Lennox furnace is short cycling, make sure you set your furnace to “heat” and “auto.”

Next, check your air filter. See instructions above on how to refresh your furnace filter.

Then, check your home for drafts and potential air leaks. See above for tips. Also, check your duct work.

Last, examine your flue. The flue is the tube that leads from your furnace to the outside of your home for ventilation. Ensure it’s not blocked or clogged by things like a snowdrift or animal’s nest.

If these Lennox furnace troubleshooting tips don’t work, you may have an issue with your flame sensor, heat exchanger, or igniter. A certified HVAC technician can inspect and potentially replace or repair these pieces if necessary.

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Lennox furnace is leaking water

If your furnace is leaking water, shut off the furnace and turn off the related breaker. Use an old towel to soak up the water. If there’s a lot, use a shop vac

If you have a whole-home humidifier attached to your furnace, it may be what’s leaking water. If it looks like that’s the water source, follow our humidifier troubleshooting guide.  

Check your furnace filter. Replace it if necessary (see above for instructions). A dirty filter may restrict airflow and cause a coil to freeze. As it melts, it may leak water.

Alternatively, your heat exchange may be creating condensation. This water typically leaves the furnace through pipes that lead to a floor drain. If you’re comfortable accessing these pipes, you can use a wire brush to clean out any clogs. 

If these fixes don’t work, call a professional. They can pinpoint the issue and get it fixed.

Lennox furnace error codes

If your furnace isn’t working, its error lights may give you a clue about what’s wrong. On most Lennox furnaces, you can remove the front cover. There will be a round glass in the bottom compartment through which you can see a red or green light.

The combination of these lights and whether they’re on, off, or flashing mean different things. See the table below to decode these error lights.

Note: “Fast flash” means the light blinks once every second. “Slow flash” means the light blinks every three seconds.

Lennox furnace error codeWhat it meansCan I fix it?
Red and green simultaneous slow flashCooling and continuous fan onN/A. Normal operation.
Red and green simultaneous fast flashHeat initiated by thermostatN/A. Normal operation.
Red and green alternating slow flashBurner failed to ignite or flame lostMaybe. Clean burners and replace air filter. Call a pro if that doesn’t help.
Red and green alternating fast flashImproper main groundNo. Call an electrician.
Either or both lights on, no flashCircuit board malfunction or incorrect wiringNo. Call a pro.
Red slow flash and green fast flashLow flame signalNo. Flame sensor rod needs replacement. Call a pro.
Red slow flash and green onLimit switch openYes. See our guide to furnace limit switches.
Red slow flash and green offGas valve not energizedMaybe. Clean flame sensor rod. Call a pro if that doesn’t help.
Red fast flash and green slow flashMain power polarity reversedNo. Call an electrician to fix improper wiring.
Red on and green slow flashRollout switch open or 12-pin connector improperly attachedNo. Call a pro.
Red off and green slow flashPressure switch issue or blocked exhaust ventMaybe. Check exhaust vent and clear. Open pressure switch. Call a pro if that doesn’t help.

Lennox furnace troubleshooting resources

Lennox has a furnace troubleshooting tool on its website that may help you identify and solve issues with your system. The company also has a product literature look-up to view the owner’s manual for your model. 

Both of these Lennox furnace troubleshooting resources may help you locate parts. They can also help you decide if it’s best to call a technician or attempt a DIY fix.

If you’ve tried all of the tips and resources above and still can’t fix the problem, consider calling a certified HVAC expert. Or, if you’re not comfortable performing any of the troubleshooting tasks, a professional can do them for you. Click below to connect with a professional who can help get your furnace back in shape. 

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