How to Troubleshoot a Lennox Furnace When Lights Flash Alternately

Find the code for your Lennox flashing lights in your service manual, on the back of the furnace or on the back of the service panel. Lennox uses different codes for different furnaces so don't assume that another model's codes will fit your model.

Do not press the reset button if one is available. If this resets the flashing light sequence, the service technician might not be able to diagnose the problem if it is intermittent. You will have to wait until the problem occurs again by causing the lights to flash.

Compare the lights and the flashing sequence to your model code. Some models come with only a red light and some with only a green. Other models have both a red and green light. Generally speaking, if a light is flashing slowly it means the furnace is on and waiting for a heat request. If the light is flashing quickly but steadily it means everything is OK, and it is providing heat. Other sequences can tell you about problems with the gas or electrical supply, a lock-out because of too many times attempting to start, or a failure in the ignition sparker.

Call your Lennox service technician if the blinking lights indicate a problem with the gas or electric. A gas furnace is dangerous when it is not working properly and it is better to have it checked than to suffer a house fire or worse.