Lennox E124

What does Lennox E124 mean?

E124 is a Service Urgent alert indicating the thermostat (or smart hub) has lost communication with a system component for more than three minutes. Unlike the transient E120, E124 represents a sustained communication loss that stops all associated system operations. The control waits for a heartbeat message from the missing component. The alert clears when communication is re-established.

Symptoms

• E124 or 'Active Sub-net Controller Missing' (or 'Tstat Lost Communication') displayed on the thermostat.
• System operation stops — no heating, cooling, or fan response to thermostat demand.
• The specific missing component name replaces the word DEVICE in the alert text.
• Alert persists until communication is physically restored.

Common causes

• Broken or disconnected communication wire between the thermostat and the missing component.
• Loose terminal connection on the i+/i- bus at the affected component.
• Float switch or isolation relay breaking the common wire to the outdoor unit.
• Miswired accessory causing false component codes across the bus.
• Power loss at the missing component preventing it from responding.

Diagnostic steps

1. 01

   Identify the missing component from the alert text

   The thermostat replaces DEVICE with the actual component name (e.g., outdoor unit, PureAir S, iHarmony). Focus diagnostic steps on that specific component and its wiring first.

2. 02

   Check wiring connections to the missing component

   Inspect i+, i-, R, and C terminal connections at the missing component for looseness or damage. A single loose i+ or i- connection breaks the entire communication path to that device.

3. 03

   Check for a float switch interrupting the common wire

   Per Lennox documentation, if a float switch is installed, use an isolation relay to break the common wire rather than connecting the float switch directly in the communication path. For testing, temporarily remove the float switch from the circuit.

4. 04

   Disconnect all accessories and reconnect one at a time

   Remove all wiring to accessories except the thermostat-to-indoor-unit connection. Reconnect one component at a time, power-cycling and checking E124 after each reconnection. The device that reintroduces E124 is the source.

5. 05

   Ohm the communication wires

   With power off, measure resistance of the i+ and i- conductors between the indoor unit and the missing component. Resistance above a few ohms or infinite resistance indicates a broken wire or corroded splice.

When to call a professional

If E124 persists after wiring checks and the missing component receives correct power, the component's communicating control board may be failed. Control board replacement on any communicating Lennox component requires system recommissioning and, in the case of the outdoor unit or furnace, refrigerant system verification by a licensed HVAC technician.