Lennox Furnace Red Light Flashing 4 Times [3 Easy Solutions]

The main culprits behind the Lennox furnace red light flashing 4 times are the malfunctioning control board, faulty thermostat, and clogged air filter. However, your furnace’s four red flashes signal means that its high-limit switch is open.

lennox furnace red light flashing 4 times

The high limit switch usually remains close. But for the factors mentioned above, it can open and create this stove’s issue. Go sharply through this troubleshooting guide, and you will definitely get the solution to your furnace’s problem.

Lennox Furnace Red Light Flashing 4 Times [3 Easy Solutions]

The factors accountable for the Lennox furnace’s four times flashing problem are discussed below, including their solutions. Let’s start:

1. Control Board Malfunction

The Lennox furnace control board doesn’t have any designated code for four red flashes. Still, this issue occurs because of the malfunctioning control board. The following factors contribute to damaging the furnace’s control board. The factors are:

  • Loose furnace wire
  • Wrong or dirty fuse
  • Blower motor’s overheating
  • Failed transistors due to power surges


Lennox Furnace control board is one of its essential parts. So when you see four red flashes on it, the first thing you should do is shut off the furnace’s main power board. Keep the power board shut for 30 seconds.

 However, in the following ways, you can solve this issue. The solutions are:

  • Check the furnace’s vibration and replace the old flexible wire
  • Clean the dirty fuse and replace it if required
  • Check the blower motor’s overheating and try to reduce it
  • Replace the failed transistors

2. Defective Thermostat

Improper thermostat setting to the furnace fan causes Lennox furnace’s four red flashes. The furnace blower will not start if you do not set the thermostat’s Fan in the ON position. The factors that cause this issue are:

  • Overheating
  • Improper installation
  • Sludge


The primary way of getting rid of this is to put the furnace’s thermostat in the “OFF” position and set its Fan at “ON.” Then press the furnace’s door switch and clockwise turn the blower. Now see whether it’s still flashing four times or not. However, the other solutions are:

  • Check and try to control the thermostat’s overheating
  • Hire a technician to install it on your furnace properly

3. Clogged Air Filter

Air filter blockage or dirty AC coil restricts the easy airflow circulation in your Lennox Furnace. As a result, it flashes four times to send you the signal that you need to clean the blockage or dirt. The following factors are responsible for this trouble.

  • Leaks, holes, or gaps in your air duct
  • Dirt, dust, or foreign object build up in the air filter
  • Polluted sparks plug


All the Lennox furnace parts work in harmony with one another. If there is any problem, it will cause the ultimate furnace problem. However, the ways to solve this issue are the following:


How do you reset a Lennox furnace?

Turn off the furnace’s power switch. Its looks like the wall light switch. Then remove the furnace’s front panel. Next, press the furnace’s reset button and turn on the power switch again.

Why is the Lennox thermostat not working?

If the thermostat stops working at first, you should check its batteries. The thermostat fails to turn on because of the dead battery in most cases. Replace the batteries, and your thermostat will start working.


Now you know which factors contribute to Lennox furnace red light flashing 4 times. Expectantly, you would now solve your Lennox furnace’s red light flashing problem by yourself.

Still, if you think the above causes and solutions are not working in your furnace case, don’t hesitate. Comment and let us know your stove’s trouble, and we will try to give you the solution.

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