Lennox Air Conditioner Error Code E124

E124 means the iComfort Wi-Fi thermostat has lost communication with the outdoor unit for more than 3 minutes. When this happens, the system stops all HVAC operations until communication is restored. The fault is typically caused by wiring issues, a loss of power to the outdoor unit, or a failed control board.

Cycling power to both the indoor and outdoor units is actually the recommended first step after checking wiring, and it can clear the error if the communication loss was caused by a transient disruption. However, if an underlying problem such as damaged wiring or a failed control board is present, the error will return after the reset. The fault must be corrected at its source for the code to clear permanently.

Repair costs vary widely depending on the root cause. If the fix is a loose wire or a tripped breaker, costs may be limited to a service call fee, typically in the range of $75 to $150. Replacing damaged communication wiring can add $100 to $300 or more depending on the length of the run. A failed outdoor unit control board is the most expensive repair, with parts and labor potentially ranging from $400 to $900 or higher.

Some causes of E124 are DIY-accessible, such as checking for tripped breakers, resetting system power, and visually inspecting low-voltage wiring connections at the thermostat and air handler. However, diagnosing a damaged communication wire or a failed outdoor control board requires electrical testing and familiarity with the system wiring, so a licensed HVAC technician is recommended if the basic checks do not resolve the fault.

When E124 is active, the Lennox iComfort system automatically stops all HVAC operations, so the system will not run in an unsafe state. The error itself is a protective response rather than a dangerous condition. That said, the underlying cause should be diagnosed and repaired promptly to restore heating and cooling and prevent any further issues with the control wiring or boards.