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Lennox Gas Furnace Troubleshooting


Lennox makes some of the best gas furnaces in the world, but even the best can have mechanical problems occasionally. This Lennox gas furnace troubleshooting guide will help you know what to do when your furnace won’t start or won’t perform as it should. With service calls from furnace repairmen costing more than $100 in most cases, doing what you can to fix the problem, or at least determine its likely cause, can save you money.

The Lennox Furnace Won’t Start

This is the most common issue with a furnace.  You come home or wake up to a cold house.  What happened?  Check the thermostat first.  Is the system in heating mode?  Is the temperature set very low, so it’s not calling for heat?  Thermostats get changed by accident often.  It’s a simple fix.

If this isn’t the problem, check the circuit breaker in the electric box to see if it has been tripped.  Also make sure the on/off switch on the furnace is in the “on” position.  If all this checks out, and your furnace isn’t doing anything at all, your thermostat may be bad.

The Lennox Furnace Won’t Fire

If you hear the inducer motor running but the furnace won’t fire, it may indicate that the vent is plugged by a bird’s nest, snow, or debris.  If the burner still won’t fire, check the gas valve to make sure it is in the “on” position.  If you are using propane, make sure you’ve got fuel in your tank.

It is also possible that the hot surface igniter (glow plug) isn’t working.  Take the cover off the furnace and set the thermostat for heat.  If the furnace tries to start but the igniter never glows hot, that’s likely your problem.  You can usually see the glow plug through a small port in the front of the combustion box.

The Lennox Furnace Won’t Stay On

Is your furnace firing but then shutting down immediately?  There are several potential causes.  First, if there’s a condensate reservoir, it may be full.  If it’s full, the pump may not be working or the drain may be plugged.  See if you can clear the drain, and if not, the pump is probably bad.  Secondly, check to see if the furnace is dirty.  If the furnace isn’t getting enough air, it will shut down. Another cause of the furnace going out too quickly could be the heat sensor.  Try cleaning it first.  It may need to be replaced.

If the furnace fires but the blower doesn’t start, its motor may be bad.  The furnace will overheat and shut down if the blower doesn’t come on.

The Lennox Furnace Control Board

If you can’t identify an obvious problem, it may be the control board. Often called the mother board because it controls many of the functions, a bad board may prevent the furnace from starting, prevent it from firing, or cause the blower not to start.  If an electrical surge has knocked out the board, there may be signs of intense heat or electrical burning, a clear sign of a bad board.


This Lennox furnace troubleshooting guide will help you know where to look. By knowing what can go wrong, you may be able to fix it yourself without an expensive service call or repair.  Even if you need to call a qualified contractor to work on your furnace, having knowledge of what can go wrong will help prevent an unscrupulous repairman from taking advantage of the situation, suggesting expensive repairs that don’t need to be done.  An informed homeowner will always make a better decision in these circumstances, and we think this troubleshooting guide to Lennox furnaces will help.

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