Lennox Furnace Error Code E124

E124 means the furnace control board has not received a valid signal from the communicating thermostat for more than 3 minutes. Lennox communicating systems rely on a continuous digital data link over a four-wire connection, and any interruption longer than 3 minutes triggers this fault. The furnace will suspend normal operation until communication is restored. The most common causes are loose wiring connections or a power issue at the thermostat.

Cycling power at the thermostat is the recommended reset step for E124 and can clear the fault if the communication loss was caused by a temporary signal disruption. However, if an underlying wiring problem or component failure is present, the code will return after the reset. Always inspect and ohm the four-wire communication wiring before relying on a power cycle as a solution.

If the fix is simply reseating a loose wire connection, there may be no parts cost involved, with a service call typically ranging from $75 to $150 depending on your area. If the communication wire run needs replacement, parts and labor costs can range from $100 to $300. A failed communicating thermostat replacement can range from $200 to $600 depending on the specific model, and a furnace control board replacement can range from $300 to $700 including labor.

Start by inspecting all four wiring connections at the thermostat and at the furnace control board, making sure every wire is firmly seated at the correct terminal. Use a multimeter to ohm out each wire individually to check for continuity and rule out a broken conductor. After confirming the wiring is intact, cycle power at the thermostat to allow the communication link to reinitialize. If the error persists after these steps, the thermostat or the furnace control board may require replacement.

E124 is a communication fault rather than a combustion or pressure safety fault, so it does not indicate an immediate danger such as a gas leak or carbon monoxide risk. However, the furnace may not operate properly while the fault is active, which means it may not heat your home. The issue should be diagnosed and resolved promptly, especially during cold weather, to restore normal heating operation.