How to Troubleshoot an Error Code on a Lennox Furnace

Diagnostic error codes can be frustrating to homeowners, but they are necessary to ensure the safety and efficiency of your heating system. The Lennox Furnace can display several diagnostic codes if there is a problem with the system. Understanding what these error codes mean and how you can troubleshoot them will keep your heater running smoothly for many more years to come. To help you diagnose your Lennox furnace, we have listed some common error codes below. In most cases, these codes signify a minor problem that is easily fixed. However, it’s important to get any problems diagnosed by an expert as soon as possible so they don’t escalate into something more serious and expensive to fix.

Error Code 1 – Blower Fan Does Not Run

The blower fan is responsible for pushing air through your home’s ductwork. One common issue that can cause the blower fan to shut down clogs in the ductwork that cause airflow to be restricted. A clog can be caused by lots of different things and maybe easy to fix. However, if ductwork is clogged, it’s likely that the problem is more severe and may require a more serious repair. If your blower fan is not running, you may also have a problem with your thermostat. The blower may not be working if the thermostat isn’t sending it a signal to turn on. Make sure your thermostat is set to “on” and that there are no low batteries. If your blower still isn’t on, your system may have an error code that requires the help of a professional.

Error Code 2 – Furnace Will Not Start

There are a few different reasons you may experience a problem starting your furnace. If the system won’t turn on at all, the most common cause is a low voltage problem. A fuse may have blown or your thermostat may have shut off power to the system. If you have a blown fuse, you can easily fix the problem yourself. If your thermostat is cutting off power, you will need to reset it. If your furnace is not starting but your blower fan is running, you may have a clog in your ductwork. This can be caused by a buildup of dust or debris, which can be easy to cleanout. If you can’t find and fix the problem, it may be an issue with your thermostat. Your thermostat may be set to “off” or experiencing a low battery issue. If none of these issues are causing problems, your furnace may have a more serious issue that should be fixed by a professional.

Error Code 3 – Furnace Fan Will Not Turn On

If the furnace fan is not turning on, you may have a problem with your thermostat. Your thermostat may have been set to “off” or have a low battery. If your thermostat is set to “off” and you’d like it to be on, simply reset the temperature. If you have low batteries, you can replace them or plug your thermostat in. If the furnace fan is not turning on but the furnace is working, you may have a clog in your ductwork. This can be caused by debris or a build-up of dust. If you have a clog, you can try to clean it out yourself. If the problem is more serious, it may require a professional to remove the clog.

Error Code 4 – Furnace Is Over-Shifting

An over-shift error means that your furnace is turning off before it should. Overshifts can be caused by a variety of factors. The most common reasons for an over-shift are low furnace pressure and high furnace temperature. Low furnace pressure is caused by a clogged filter or dirty burners. High furnace temperature can result from a lack of airflow, a restricted duct, or a clog in the duct. To fix an over-the-shift, you may need to clean your furnace filter, clean your ductwork, or have a professional remove any clogs.

Error Code 5 – Incorrect Temperature Reading

If you notice that the temperature on your thermostat is not matching the temperature in your house, you may have a problem with your temperature sensors. You can manually calibrate your thermostat or replace your sensors. If you don’t want to touch your thermostat, you can manually calibrate it by adjusting the two screws on the back of the unit. If you notice that the temperature reading is incorrect but your thermostat is not being used, you may have a problem with your wiring. The wires may be crossed, which can easily be fixed. If you have a new thermostat and are still experiencing incorrect temperatures, you may need to call an electrician.

Error Code 6 – Furnace Shifting Too Soon

An early shift error occurs when your furnace turns on before the temperature reaches the thermostat setting. A furnace that shifts too soon can cause your house to overheat, which can be dangerous and expensive. If you experience an early shift, you first want to check the temperature reading to make sure it’s accurate. If the temperature reading is correct, try adjusting the thermostat temperature setting to see if that helps. If you’ve already tried that and are still experiencing an early shift, it may be an issue with the thermostat.

Error Code 7 – Furnace Will Not Shut Off

If your furnace will not turn off, the first thing you should check is the temperature reading. If the temperature is set to off and the furnace is still running, you may have a problem with the thermostat. Before you replace your thermostat, make sure you are familiar with the error code and what needs to be done to fix it.


The furnace is a huge part of any home, so it’s important to be able to troubleshoot any potential issues that could arise. Knowing what each error code means and how you can fix it will save you lots of time and money by preventing bigger problems from developing. Now that you know what each error code means, you’ll be able to quickly identify and fix issues before they become serious problems.