Lennox Furnace Error Codes – Troubleshooting Guide

Are you assuming that there is something wrong with your furnace? Don’t worry, we have got you covered!

A Lennox furnace has two LED lights that the system uses to say that something is not right. The lights can flash at different frequencies, be switched off, or steadily on – each combination is a different error code that indicates the problem.

Most Common Lennox Furnace Error Codes

Where can you find these error codes?

Locate the front cover of your unit and remove it. There, you will see two compartments – the top and the bottom.

The bottom compartment has see-through glass. This is exactly where you can check if your furnace is trying to tell you something. 

Make sure to take note of the flashing sequence and then find a description below that corresponds to it.

Read: American Standard Furnace – Troubleshooting Guide

Both LEDs are off

The furnace isn’t receiving power. 

Your unit might simply be turned off. 

The furnace’s power switch is usually located on the actual unit or somewhere next to it (the thing looks like a regular light switch). Flip the switch to the ‘on’ position, if it was off.

Tip: the chances are high that the furnace won’t start operating immediately. Give the system at least a few minutes. 

You can also go to the main electrical panel and find your furnace’s circuit. The circuit breaker might have tripped, so flip it to ‘on’.

If the furnace continues to trip the circuit breaker – better call a professional. 

Both LEDs are blinking slowly

There is an issue with the blower motor. 

First things first, check if your blower motor is receiving power. Perhaps, the components had burned out.

Inspect the blower fan – you can try spinning the blades with your hand. If they don’t spin freely, then you would have to replace either the fan or the whole motor.

Here are a few other things that you can do:

  • Inspect the wiring – make sure that there are no loose or damaged connections (sometimes, unplugging the fan for a few minutes and then plugging it back in may help).
  • Check the thermostat – the device has to be working correctly.
  • Inspect the vents and filters – the arrows in the filter should be pointing in the right direction and there shouldn’t be any obstructions in the system.
  • Check the blower motor belt – it might be cracked or worn down.

Both LEDs are blinking slowly one at a time

The flame system had failed.

The chances are high that the flame sensor is dirty

  • Before attempting to clean the sensor, turn the furnace off.
  • Remove the sensor. It is usually mounted with a screw; once you remove it, the flame sensor is going to slide out.
  • Gently rub the metal rod with steel wool, a wire brush, or very light grain sandpaper.
  • Put the sensor back in place and check, if the problem had gone.

This error code might also occur if there is an issue with the gas valve or the ignitor. 

Both LEDs are blinking quickly one at a time

The furnace is not receiving the proper level of voltage. 

Fact: a lot of units run on 120 volts, but the majority of those have a transformer that ‘turns’ the 120 volts into 24. Such power is used for the fan timer and the thermostat, for example.

Check if your house is receiving ample voltage. There might also be a loose connection somewhere in the system. 

The error may indicate that there is an issue with the ignitor. If the breaker hadn’t tripped and the connections are in place, then calling an HVAC expert might be the best decision.

First LED is blinking slowly, the other is continuous

The high limit switch is open. 

The chances are high that your furnace had overheated. The limit switch turns the burner off and turns on the fan.

Once the temperature gets back to normal, the burner will be switched back on.

The most common root causes of such a problem are a clogged filter and a dirty blower wheel. 

Read: Why Gas Furnace Is Overheating?

First LED is blinking slowly, the other is off       

The unit was unable to detect gas.

It looks like there is a problem with the ignitor, gas valve, or gas supply.

These are the things that you can check on your own:

  • Check the wires to the gas valve – they should not be damaged.
  • Check the flame sensor – if it’s dirty, you can attempt cleaning the component.
  • Inspect the evaporator coil – a dirty coil might make the inside of the system get too hot, in such a case, the limit switch is going to trip and shut off the unit.
  • Visually check the return duct – if it is crushed or damaged in any other way, it will affect the airflow.

A professional is, most likely, going to check the coil’s resistance, the gas pressure, and will replace any wires, if there is such a need. 

First LED is off, the other is blinking slowly

A faulty draft inducer or a closed pressure switch circuit. 

A draft inducer is, basically, a fan that is responsible for clearing out any combustion gases. It is one of the first components that turn on during the heating cycle and it keeps operating until the very end.

The draft inducer works together with the air pressure switch.

If the latter detects too much air blowing through the heat exchanger or no air at all, it will shut the whole system down. 

This is definitely an issue that you should leave to a professional, as the components are extremely important safety devices. 

Read: Why Does Air Conditioner Run At The Same Time As A Furnace?

First LED is flashing slowly, the other is blinking quickly

The flame level is low.

At times, all you would have to do is clean the flame sensor. Don’t forget to shut off the power to the unit, before making any changes.

Warning! If the gas valve on your furnace is not electronically controlled, you would have to shut off the gas to the system as well.

You can also clean the place where the gas shows up (usually, a pilot nozzle) with a wire brush.

Inspecting the quality of the flame is another thing that the homeowner can do on his own. The furnace should be producing straight, blue flames.

A yellow flame can, in a lot of cases, be ‘fixed’ by cleaning the components mentioned above.

Finally, you can adjust the height of the pilot flame

There is a special adjustment screw that you can twist to make the flame go to about 2 inches. If your unit has such a component, it will be close to the gas valve, on the pilot assembly.

First LED is flashing quickly, the other is blinking slow

The polarity of the furnace is reversed.

Warning! Don’t try to fix this issue on your own, if you don’t feel comfortable working with wires. 

The polarity can become reversed if someone changed the wiring at the breaker box or in the circuit. ‘Reversed’ means that the neutral wire is connected to where the hot wire has to be.

A furnace can operate on reversed polarity for years, but, at one point, the unit will start having an issue with this mistake.

But the worst thing is that a furnace with a reversed polarity stays energized even when it’s off as the electric current can flow backward.

The only way to deal with the problem is to check the wiring and ensure that every wire is connected the way it should be.

Read: Why Is My Furnace Beeping?

One LED is blinking, the other is steady

The flame rollout switch is open. 

A furnace’s flame rollout switch is a safety component that is there to detect a fire inside the burner compartment (if there ever is one).

This switch looks like a small metal disc with 2 wires. It is usually located on the top or the side of the burner compartment.

Here are a few things that can be causing this error:

  1. A clogged exhaust vent
  2. A clogged or a cracked heat exchanger
  3. Low gas pressure
  4. A faulty flame rollout switch

During proper operation, the flames created by the burners get ‘sucked in’ the heat exchanger. But that might not happen and the flames will start to roll out.

In such a case, the inside of your furnace can get burned by the flames. That’s when the switch takes center stage and trips.

Read: Why Does My Furnace Filter Get Dirty So Quickly