Lennox Furnace Troubleshooting The Definitive Guide

Lennox Furnace Troubleshooting: The Definitive Guide

If you’re anything like us, your home is your sanctuary. You want to relax and unwind in the comfort of your own space. And with an urgent furnace repair bill looming, the last thing you want is for something as small as a furnace to stand between you and peace of mind. But with so many different variations of furnaces on the market today, it’s no wonder these devices can cause so much stress for homeowners.

Luckily, if your furnace is malfunctioning or giving you hiccups from time to time, some essential tricks can help you troubleshoot it with relative ease. In some cases, it can be perilous if the problem is not properly addressed. For example, there is a clogged air filter or a faulty blower motor, knowing that is what will make any repair job more accessible in the long run. Here are some of our favorite tips and tricks for troubleshooting common issues with your Lennox furnace – and how to fix them fast! Keep reading this article. 

Lennox Furnace Problems

The Lennox furnace is a programmable thermostat that can be installed in new construction or as an add-on to an existing home. The Lennox furnace comes in two types: condensing and non-condensing. Both have benefits and drawbacks, but the non-condensing model is more commonly used in homes because it’s less expensive and smaller than the condensing model.

Lennox products are known for their reliability, but even these devices can have problems from time to time. Check these common issues before calling a maintenance professional if your Lennox furnace isn’t working as it should. Doing so could save you from spending unnecessary money on repairs or replacement:

Lennox Furnace Error Codes

The furnace’s top and bottom parts are accessible once you remove its front cover. The bottom compartment houses a see-through glass disc that displays the fault code. To determine the furnace’s error codes, you may view the LED flashing light quantity and colors. Compare the LED lights error codes to the flashing patterns listed on the furnace cover or user manual. This may have links to the furnace problem.

Inducer Motor Problem

When the thermostat demands warmth, the inducer motor comes on to guarantee there are no restrictions on exhaust and intake vents. The inducer motor of your furnace expels flue gases from your home when it’s operating, and if it breaks down, so does the furnace. Locate the inducer motor on the top box of the furnace. You can test the inducer motor by touching it. As long as it’s too hot, it’s burnt. Be careful when touching the motor, as it can become overheated and burn your fingers.

Thermostat Problem

Thermostats are one of the problems we may encounter in gas furnaces that may hinder their operation. Thermostats are devices that control the temperature of the furnace. It uses batteries, which can be AA or AAA. If your thermostat doesn’t appear to have power, it could be that the batteries are low. Replace these to see if they work correctly. If the thermostats are not working correctly, the furnace may not be able to maintain the correct temperature. It can be a lot of problem because the furnace may not be able to heat the home properly.

Ignition Problem

  • Standing Continuous Pilot: A pilot light that won’t stay lit frequently plagues an older heater. These heaters lack an inducer motor, so the gas valve opens when the system requires heat and sends gas to the burners. The inducer motor keeps a small flame burning many times, which lights the main burners when gas is sent to them.
  • Spark Igniter: You may ignite high energy star furnaces using an ignitor. Once the inducer motor starts, the circuit board sends a signal to the ignitor, which sparks the burners. After lighting the burners, the igniter stops sparking.
  • Hot Surface Igniter (HSI): The most common type is a furnace with HSI. Once the inducer motor is activated and the circuit board sends a signal to HSI, it will light the burners. If the furnace is operating correctly, HSI will glow when the main burners are activated and stop glowing once they are lit. If the pilot light is visible, HSI glows, or you can hear the sparks, but the burners don’t ignite, then a gas valve, circuit board, or gas-related issue is likely the culprit.

Furnace Filter

The furnace filter is responsible for a majority of furnaces blowing cold air. Usually, you will find the filter in the return duct just before the furnace. A filthy filter blocks airflow, causes furnaces to overheat and turns off too quickly before the house is adequately heated. You should change your furnace filters every two to three months. In this way, you can stop this problem from occurring. 

Clogged Drain

Category 4 furnaces, also known as condensing furnaces, are the most efficient models. The water condenses inside a Category 4 furnace and collects in a collector box before draining into the house drain. Make sure to suck the drain with a vacuum to unblock it. Vacuuming the drain with a shop vacuum will remove any blockages. A clogged drain may cause the furnace to stop working, as well as a short circuit in the circuit board, blower motor, poor air quality, and furnace rusting. Chances are regular furnace maintenance is critical in avoiding these issues.

Main Circuit Breaker Problem

Make sure the furnace has power before you begin working on it. If the breaker is off, you can turn it on again. If the breaker is in the middle, move it to the off position. It will reset the breaker. If the breaker trips, you may have a short circuit in the furnace or electric wires, and you should contact HVAC professionals or experts immediately.

Furnace Vents

Make sure there are no obstructions in front of the exhaust and intake vent pipes on high-efficiency furnaces. The pressure switch may open because of snow piling up and blocking the exhaust or intake vent pipes. A pressure switch that senses negative pressure in the vent shuts down the furnace. Vent pipes must be free of blockage.

Flame Sensors Problem

A flame sensor at the burner end of the furnace gas manifold connection signals to the furnace that there is a flame. Without it, the furnace would not know whether a flame was present. Furnaces often have short cycles because of flame sensor failure as well. The circuit includes a flame sensing rod in front of the last burner.

Reasons for low or no flame current:

  • Dirty flame rod
  • Someone has cracked a damaged ceramic insulator on a flame sensing rod.
  • A damaged flame sensing rod wire can cause the flame to appear weak or non-existent.
  • Poor furnace ground
  • Debris emitted from the furnace.
  • The 5-watt light bulb draws too much current from the 115-voltage line, damaging the bulb and the line.
  • Low gas supply

Lennox Furnace Flash Codes

A slow flash rate equals one flash per second (1 Hz). A fast flash rate equals three flashes per second (3 Hz). The minimum flame sense current is 0.5 microAmps.

Reference: https://www.airtronic.ca/lennox-furnace-troubleshooting/

Steps to Reset a Lennox Furnace

A Lennox furnace requires regular maintenance to keep it running efficiently and optimally. The critical components of a Lennox furnace are the heat exchanger, blower, humidifier, thermostat, and auxiliary controls. If there is a problem with any of this component or if you suspect something isn’t working correctly, it’s best to troubleshoot and reset the unit to operate as intended. Follow these steps to reset your Lennox furnace:

1. Turn off the power switch to the furnace

The furnace’s control panel will have a power switch on its side, which looks like a wall light switch. Make sure to turn off the circuit breaker switch corresponding to the furnace.

2. Remove the front panel of the furnace

Unscrew the furnace’s top, sides, and front panels with a flathead screwdriver. Lift the front panel off and set it aside afterward. Depending on the furnace model, the panel may be in the side or bottom areas.

3. Find the reset button

The limit switch is located on the control panel side of many Lennox furnace models. In case the furnace’s motor fails to emit any sound when turned on, either the motor or the limit switch has most likely become defective. Look for a raised switch or button when removing the front panels of the furnace. If you are unsure how to recognize the switch, refer to your user’s manual. Reactivate the furnace by pressing and releasing the reset button.

4. Turn the power back on

Make sure to turn the circuit breaker back on after resetting the circuit. Then, turn the furnace switch back on. If the reset button fixes the issue, you’re good for going. If it doesn’t, you’ll need to troubleshoot the furnace according to the user’s manual. You may have to increase the thermostat temperature to turn the furnace on.


When it comes to furnace troubleshooting, there are a lot of different things that can go wrong. 

For example, if the furnace is not getting enough electricity, it may not be able to operate correctly. In this case, you may need to check the circuit breaker or the fuse box. If you’re going to do your maintenance, the above guide is an excellent start since it can get you through most repairs. Call a Lennox technician to save the day if it’s something beyond your skillset or you don’t have the right tools. 

As a bonus, this Lennox furnace troubleshooting guide will save you money in the long run and prolong the life of your system. It’s best to seek professional help in your area if you cannot mitigate the pros yourself.


What does a red blinking light on my Lennox furnace mean?

When your furnace blinks red repeatedly, you know something is wrong. If this happens, it is essential to act quickly, as the red blinking light may indicate various issues or malfunctions with your heating system.

What should I check when a furnace is not working?

You can check a few things when your furnace is not working. First, check to see if you lit the pilot’s light. If it is not, you may need to relight it. Second, check if you set the thermostat to the correct temperature. Check for a power outage, furnace filter, or furnace blower.

How do I fix a beeping furnace?

The circuit breaker might be the source of the issue, so check if it’s flipped to the ‘off’ position. If the furnace isn’t working, you may have to reset it. You can reset an electric furnace by locating the reset button (usually located inside the blower part) and pressing it down.