Lennox Furnace Problems You need To Know

Lennox furnaces are a workhorse. However, you may encounter the symptoms of a few typical Lennox furnace problems like inadequate heating, ignition or burner issues, blower not running or operating continuously, frequent starts & stops, abrupt shutdowns, and loud noise in the unit. 

The most common Lennox furnace problems are thermostat issues, dirty or cracked flame sensors, malfunctioning gas valves & pressure switches, clogged inlet and exhaust vents, jammed condensate lines, failing draft inducer motors, or a bad circuit board.

So, as you can see, there are quite a few things that could be going wrong with your Lennox furnace. I’ll tell you about the most common issues with these furnaces and teach you how to fix them for a fully-functional heating system. 

1. The Lennox Furnace Thermostat Is Faulty or Malfunctioning 

A Lennox furnace not heating adequately is not necessarily the unit’s fault. 

Your Lennox furnace thermostat may have a faulty sensor, the wiring could be frayed, or it may have lost its power connection. 

If you detect a problem with your furnace, begin a routine inspection by checking power availability and gas supply. 

Review the thermostat settings, including the mode, temperature, and blower. If you don’t find anything amiss on your thermostat, go down to the Lennox furnace, open the front cover and check the LED lights through the glass peephole. 

These lights may flash, glow, or blink if there is an error or clog in the system. So, if you see them, refer to the error codes on the panel or manual to diagnose the problem

If your Lennox furnace has no power, check the circuit breaker to see if it is tripped. You may reset the circuit breaker and restart the Lennox furnace, but exercise caution if you suspect a short circuit. If the circuit has shorted, always call a certified HVAC technician and don’t operate the furnace. 

Read: Lennox Furnace Error Codes – Troubleshooting Guide

How To Fix?

A faulty temperature sensor necessitates thermostat replacement. Change frayed, damaged, or loose wires disrupting continuity. However, if you cannot decipher any thermostat problem and the Lennox furnace has no other evident issues, you must opt for a professional inspection. 

Have a Question? Ask HVAC Technician

Click here to use the chatbox to speak with one of our technicians.
No in-home service calls. No appointments.

Read: Why Is My Furnace Beeping?

2. The Flame Sensor Is Dirty, Cracked, or Disconnected

If your furnace has a dirty or cracked flame sensor, it can quickly start and shut down (also called short-cycling). 

Most modern Lennox furnaces try to restart up to 5 times for a particular heating cycle. Your Lennox furnace will stop working if the flame sensor isn’t doing its job. 

When this happens, the LED display on your furnace will show a slow flash of the red light with a simultaneous slow flash of the green light. 

This flashing pattern indicates that the burner lost ignition or the sensor did not detect any flame after the furnace’s five attempts to get started. The most common problem is a dirty or cracked sensor. Check the flame sensor inside the burner chamber, a rod atop the last burner across the igniter. 

How To Fix?

Clean a dirty or ash-covered flame sensor with a soft, clean rag. Inspect the wire connecting the flame sensor, as it should not be frayed or loosely coupled. Also, look for cracks and other signs of damage on the sensor. If the rod’s ceramic insulation is cracked or damaged, you will need to replace the component. 

Read: Does Furnace Filter Size Matter?

3. The Gas Valve and Pressure Switch Are Not Working

Lennox furnaces starting and shutting down or not working may be due to a failing gas valve or unresponsive pressure switch. 

Ensure the furnace has power and there is sufficient gas supply. Check the pressure line connecting the switch with the draft inducer motor chamber. Finally, use a manometer to inspect the pressure at the inlet and outlet terminals regulated by the gas valve

The pressure line is a hose, and it can get clogged, thus preventing the switch from closing to allow ignition and sustained burner operation. Likewise, the valve regulating the natural gas or propane supply may be broken, thus shutting down your Lennox furnace and its burners. 

How To Fix?

To start, you should clean the pressure hose and replug it. Then, you will need to check the gas valve. Inspecting a faulty gas valve requires a pressure gauge or manometer. Natural gas pressure should range between 7 and 11 WC on a digital manometer. Propane or liquefied petroleum gas pressure should be between 10 and 12.5 WC.

Also, you cannot switch from natural gas to propane without changing the spring inside your gas valve. The springs and valves are slightly different for the two gasses since they have different regular pressures. Don’t take apart Lennox furnace components unless you are trained or experienced in furnace repairs.

Read: Why Lennox Furnace Not Igniting?

4. The Inlet and Exhaust Vents Are Clogged or Blocked

Lennox furnaces overheat due to low airflow into the unit or blocked exhaust vents. 

When this happens, the system will shut down to avert a hazard. Check the inlet and exhaust vents. You will have a return duct inside your house and an exhaust vent that eventually leads outside. 

How To Fix?

To fix a clog in the vents, remove all obstructions from the inlet and exhaust vents. A Lennox furnace must get adequate airflow for combustion, and the exhaust fumes should not build up inside the furnace or the supply duct.

You should also check if the supply vents are closed in any part of your house and the position of the dampers. Closed vents or dampers may overheat the unit, thus shutting it down. 

5. The Draft Inducer Motor Is Failing or Not Working

All Lennox furnaces have a draft inducer motor, usually inside the top compartment. A failing or malfunctioning draft inducer motor will not draw sufficient air for the furnace to heat up, and the fuel gasses won’t make it through the exhaust duct. 

Failing draft inducer motors don’t close the pressure switch, so they stop the ignition process. Hence, your Lennox furnace will stop.

How To Fix?

When there isn’t enough air circulation, the draft inducer motor burns out, so you may need to replace yours. Exercise caution as you touch or try to sense if the motor housing is unusually hot. An optimally functioning draft inducer motor should not be too hot or loud. You will hear a normal buzzing noise, and the burners will operate expectedly.

Also, a Lennox furnace will stop working due to a faulty motor because of insufficient airflow inside the burner chamber for combustion. Call a certified HVAC technician to replace the motor. 

Read: Furnace Blower Motor Makes Noise When Starting – Troubleshooting Guide

6. The Circuit Board Is Broken or Disrupting Continuity

You have a broken Lennox furnace circuit or control board if the red and green LEDs are on, the red is off and green is on, or if red is on and green is off. Check the error codes for your model to verify if there is an electrical continuity problem. Also, check if the furnace circuit has reversed polarity. 

Reversed polarity is a prevalent issue with most furnaces, not just Lennox. Essentially, you have a wiring problem wherein the neutral and hot terminals are interchanged. Lennox furnaces can operate despite reversed polarity, so you may discover this issue, even if the cause of your furnace problem isn’t polarity.

How To Fix?

A defective circuit board needs replacement. Continuity issues need an extensive diagnosis to detect precisely where you have a wiring issue or control board failure. If the circuit board is working and there is no continuity problem, you can troubleshoot reversed polarity. 

Watch this video tutorial to fix reversed polarity:

7. The Condensate Line and Drain Are Jammed or Frozen

Like the clogged or blocked inlet and exhaust vents, a frozen drainpipe and jammed condensate line may shut down your Lennox furnace. Also, unchecked water and ice buildup will cause rust and other issues in your furnace, so a proactive diagnosis is always better. 

How To Fix?

You can drain excess water from the condensate line and pipe into a bucket to clear the line. Then, use a vacuum to eliminate any blockage or debris clogging up these lines. Also, ensure that the condensate line and drain pipe, in particular, do not freeze during extreme winter nights. If freezing is an issue, provide the pipes with insulation, coating them in foam or rubber. 

8. One or More Lennox Furnace Components Are Defective 

You may have a defective igniter, flame rollout limit switch, blower, or capacitor. Also, the furnace filter may be dirty and clogged, thus preventing airflow into the heat exchanger of your Lennox furnace. Any defect beyond remedial cleaning calls for immediate repair or replacement.  

How To Fix?

You may need to replace the igniter, capacitor, or flame rollout limit switch depending on the problem. 

Lubricating a blower or belt may reduce the loud noise and increase its efficiency unless the motor fails. Likewise, change the inlet filter to increase airflow if other problems don’t exist, such as a broken draft inducer motor not drawing enough air into the heat exchanger. 

Still, these issues are usually best to leave to the pros since a mistake could further damage your furnace. 

Read: Lennox Furnace Problems


Almost all typical symptoms of Lennox furnace problems are due to the components discussed in this article. Whenever you have an issue, refer to the error codes, inspect the corresponding part as detailed in this guide, and opt for the appropriate remedy based on its condition.