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Lennox Furnace Codes

How to Fix Lennox Furnace Codes

Are you having trouble with your Lennox furnace? Do you see some Lennox furnace codes flashing on the control board? Don’t worry, we are here to help you. Heating, Ventilation,…

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Are you having trouble with your Lennox furnace? Do you see some Lennox furnace codes flashing on the control board? Don’t worry, we are here to help you.

Heating, Ventilation, and Air Conditioning (HVAC) systems play a crucial role in providing a comfortable indoor environment. Among these, the furnace is a fundamental component that ensures warmth and coziness in your home during the chilly months. 

And the Lennox brand is a forerunner in the industry, offering high-quality, energy-efficient heating systems for both residential and commercial applications. However, like all HVAC systems, your Lennox furnace may encounter issues from time to time, denoted by certain error codes, often referred to as “Lennox Furnace Codes.” 

In this guide, we will explain the meanings of these codes and how to troubleshoot them to ensure the smooth operation of your furnace.

What are Lennox Furnace Codes?

Lennox furnace codes are alphanumeric codes that indicate the status of your furnace.

Lennox Furnace models come equipped with advanced diagnostic capabilities. They display error codes through an LED light sequence to alert the user or the technician about operational issues within the unit. 

These Lennox Furnace Codes are critical in troubleshooting and resolving problems. They serve as a signal for you to investigate and possibly call for professional help.

For instance, Lennox Furnace code A125 indicates a low flame signal, which could stem from dirty flame sensors, low gas pressure, or a malfunctioning flame sensor. It is always advisable to refer to your Lennox Furnace manual for a complete list of error codes, as each model may differ slightly.

READ: How to Fix Payne Furnace Codes 3 Flashes

Common Lennox Furnace Codes and Troubleshooting Techniques

Code: E200 (Lower Pressure Switch Didn’t Close)

This error code suggests that the lower pressure switch did not close as expected. This could result from blocked vent pipes, causing an abnormal pressure difference. First, inspect the vent pipes for any obstructions and clear them if necessary. 

If this doesn’t resolve the issue, it might be due to a defective pressure switch. In this case, it would be best to consult a professional.

Code: E228 (Furnace Blower Running Continuously)

Code E228 implies that your furnace’s blower is running non-stop for more than 10 minutes without any call for heat. This could be due to a thermostat issue, or a malfunction in the control board.

To address this, you can try to reset the unit by turning off the power for a minute and then turning it back on. If the blower still doesn’t stop running, it would be advisable to seek professional help.

Code: A125 (Low Flame Signal)

As mentioned earlier, the A125 error code signifies a low flame signal. This might be caused by a dirty flame sensor, low gas pressure, or a defective flame sensor. The flame sensor is a safety device that checks whether the furnace is igniting correctly. 

To address this issue, try cleaning the flame sensor with a soft cloth and ensure your gas supply is normal. If the problem persists, it could mean a defective sensor or gas valve that requires a professional to inspect and possibly replace.

Code: E202 (Upper Pressure Switch Did Not Open)

This code indicates that the upper pressure switch did not open as expected. This could be due to a malfunctioning pressure switch, issues with the vent, or a blocked condensate hose. First, ensure that the vent and the condensate hose are not blocked or damaged. 

If this doesn’t resolve the issue, you may need a professional to inspect the pressure switch and other components.

Code: E273 (Communication Error with the Control Board)

The E273 code indicates a communication error with the control board. It could be due to a wiring problem or a faulty control board. Try resetting the unit by disconnecting the power, waiting for a few minutes, and then restoring power. If the issue continues, you should contact a professional, as the control board might need to be replaced.

Code: A345 (Low Heat Pressure Switch Did Not Close)

This code signifies an issue with the pressure switch, typically caused by a blockage in the venting system or condensate drain. To address this, inspect the vent pipes and condensate drain for obstructions, and clear them if necessary. If the problem persists, consider consulting a professional, as the pressure switch might be faulty and require replacement. To understand the role of a pressure switch, you can read this comprehensive guide.

Code: A720 (High Fire O2 Level Out of Range)

The A720 error code denotes that the O2 level in high fire is out of the expected range. This could be due to issues with the combustion process, which might be caused by a blocked burner, incorrect gas pressure, or a malfunctioning gas valve. 

You might need to clean the burners, adjust the gas pressure, or have the gas valve replaced. A qualified HVAC technician can help with this.

READ: How to Troubleshoot Goodman Furnace Codes

How to Read Lennox Furnace Codes

Decoding Lennox furnace codes is an essential skill for both homeowners and HVAC professionals alike. These error codes provide valuable information about the functioning status of your Lennox furnace. Here’s how to read these codes:

  • Locate the Control Board: The error codes on a Lennox furnace are displayed on the control board, which is typically housed inside the front panel of the furnace.
  • Check the LED Light Sequence: Through the window on the control board, you will see LED lights that flash in specific sequences. Each of these sequences corresponds to a particular error code.
  • Interpret the Code: Use your Lennox Furnace manual to match the LED sequence to its corresponding error code. For instance, an LED flashing once per second denotes normal system operation, while continuous flashing indicates that the system is in heating mode. Codes that signal specific issues usually have unique light patterns, such as two short flashes followed by four long ones.
  • Troubleshoot the Issue: Once you have identified the error code, you can use the information provided in the manual or online resources like this one to understand the problem and its potential solutions. 

Understanding how to read Lennox Furnace Codes is a proactive way to keep your furnace running smoothly. 

If a code pops up, you’ll be able to respond appropriately, either by addressing the issue yourself or contacting a professional HVAC technician. However, some issues may be simple enough for you to fix yourself, such as cleaning a dirty filter or resetting a tripped breaker.

READ: How to Troubleshoot American Standard Furnace Codes

When Should You Call a Professional?

While some minor troubleshooting can be done on your own, most Lennox Furnace codes require expertise that only a certified HVAC technician can provide. If you’re unsure about a particular error code or if the troubleshooting steps do not resolve the issue, it’s advisable to reach out to a professional. 

Attempting to resolve complex issues on your own could lead to further damage, or pose potential safety risks.

Regular Maintenance

Preventive maintenance is key to minimizing the occurrence of Lennox Furnace codes. This involves scheduling regular inspections and tune-ups with a professional. Annual maintenance can help keep your furnace running efficiently, increase its lifespan, and possibly prevent small issues from turning into larger, costlier problems.

Additionally, preventive measures include changing the filters regularly, ensuring proper ventilation, and keeping the furnace and the surrounding area clean and free of debris. 

Regularly cleaning and maintaining your system can reduce the risk of malfunctions and, subsequently, the appearance of error codes.

What’s the Role of Lennox Furnace Codes?

It’s important to remember that Lennox Furnace codes are designed as a proactive measure to prevent further damage to the system and ensure optimal performance. They act as a first line of defense, allowing you to catch issues early and react accordingly. Understanding these codes can help you maintain your system effectively and ensure its longevity.

Frequently Asked Questions for Lennox Furnace Codes

Q. How do you read Lennox codes?

A. Reading Lennox codes involves interpreting the LED light sequence displayed on the control board of your Lennox furnace. These codes provide information on any operational issues within the unit. For instance, an error code like A125 indicates a low flame signal. 

Always refer to your Lennox Furnace user manual for a complete list of error codes, as the LED patterns may vary slightly between different models.

Q. Where is the error code on a Lennox furnace?

A. The error code on a Lennox furnace is displayed on the control board, usually located inside the furnace’s front panel. The control board has a window through which you can observe the LED light sequence. 

This LED pattern represents specific error codes that help diagnose operational issues with the unit. Make sure to refer to your furnace manual to match the LED sequence with the respective error code.

Q. How do you reset the code on a Lennox furnace?

A. To reset the error code on a Lennox furnace, you first need to correct the underlying issue causing the error. Once this is done, you can reset the furnace by turning off the power supply to the unit, waiting for a few minutes, and then turning it back on. Some models may have a reset button on the control panel that you can press to clear the code. Always refer to your user manual or consult with an HVAC professional for specific instructions.

Q. What is code 11 on a Lennox?

A. The error code 11 on a Lennox furnace signifies ignition failure. This could be due to various reasons, such as a faulty ignition control, a dirty flame sensor, low gas pressure, or an issue with the draft inducer motor. 

Addressing this issue may require cleaning the flame sensor, checking your gas supply, or in some cases, calling an HVAC professional for a more thorough diagnosis and resolution.


Lennox Furnace codes are not just flashing lights and numbers; they are integral to the self-diagnostic features of your HVAC system. By understanding these codes, you’re empowered to respond appropriately to potential issues, ensuring timely troubleshooting and maintenance, leading to optimal furnace performance and extended system lifespan. 

Always remember to contact a professional when in doubt, as your safety and comfort should not be compromised.

HVAC aficionado, keeping temperatures just right and comfort levels off the charts. Connect with me on LinkedIn for cooling insights and follow me on Twitter for heating up the HVAC conversation. Let's create the perfect climate together!

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Hi there, I'm Robert Brooks

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