Lennox Gas Furnace Troubleshooting
Lennox gas furnaces are known for their dependability. In the rare case that your unit isn’t working like it should, this Lennox gas furnace troubleshooting guide will help you better understand the underlying causes so you can resolve issues quickly and easily.
Any testing or repairs outlined in this guide will need to be completed by an authorized technician.
Furnace Won’t Start
- Unit isn’t turned on - Turn the unit on.
- Circuit breaker is off or tripped/fuse is blown - Turn the circuit breaker on. If issues persist, contact a licensed electrician for assistance.
- Gas supply is insufficient - Contact the gas company to remedy the issue.
Furnace Isn’t Blowing Hot Air/Is Blowing Cold Air
- Igniter is faulty - Call a licensed technician to remedy the issue.
- Blower motor is defective - Contact an authorized technician to address the issue.
- Thermostat is set to “fan” or “on” instead of “auto” - Set the thermostat to “auto.”
- Thermostat is set too low - Increase the setting to a warmer temperature.
- Filter is dirty - Replace the filter.
- High limit switch is faulty - Contact an authorized technician to address the issue.
- Pilot light isn’t on - Call a licensed technician for service.
- Burner isn’t lighting - Reach out to an authorized technician to correct the issue.
- Condensate lines are clogged - Schedule a service appointment with a licensed technician.
Furnace Is Blowing Too Much Air
- Control board is defective - Contact an authorized technician to address the issue.
- Thermostat is faulty - Call a licensed technician to remedy the issue.
- Filter is dirty - Replace the filter.
Furnace Won’t Turn Off/Is Running Continuously
- Filter is dirty - Replace the filter.
- Thermostat is set to “on” instead of “auto” - Set the thermostat to “auto.”
- Building is drafty - Make sure all windows are closed so the unit can maintain a constant indoor air temperature as easily as possible.
- Ductwork is clogged or leaking - Get in touch with an HVAC professional to remedy the issue.
Furnace Is Short Cycling (Turning On and Off Repeatedly)
- Thermostat isn’t set properly - Set the thermostat to “heat” and to the correct temperature.
- Thermostat is faulty - Schedule a service appointment with a licensed technician.
- Filter is dirty - Replace the filter.
- Blower wheel is dirty - Contact an authorized technician to service the unit.
- Air supply vents are closed or blocked - Make sure all air supply vents are open and unobstructed.
- Building is drafty - Close all windows to prevent cold air from entering the building, making it more difficult for the furnace to maintain a constant ambient temperature.
- Flue is clogged or blocked - Call an authorized technician to address the issue.
- Furnace is too big for building - Reach out to a licensed technician to correct the issue.
Furnace Is Cycling Irregularly
- Thermostat is defective - Schedule a service appointment with an authorized technician.
Thermostat Isn’t Working
- Batteries are dead - Replace the batteries. If the thermostat still doesn’t work, call an authorized technician for service.
- Thermostat is defective - Contact a licensed technician to remedy the issue.
Burner Lights Then Goes Out
- Flame sensor is dirty or defective - Contact a licensed technician to address the issue.
Furnace Is Leaking Water
- Drain pipe is clogged - Get in touch with a licensed technician to correct the issue.
- Humidifier is leaking - Contact an authorized technician to remedy the issue.
- Filter is dirty, resulting in a frozen coil - Replace the filter.
- Heat exchanger is clogged, resulting in condensation - Schedule a service call with a licensed technician.
- Ductwork insulation is poor, causing a leak from the air ducts - Call an authorized technician to remedy the issue.
- Condensate trap is blocked - Reach out to a licensed technician to address the issue.
- Condensate pump is damaged - Contact an authorized technician to correct the issue.
For more specific information about your Lennox gas furnace, the large library of Lennox manuals on our site is a great resource.