Lennox Error Code 124: Thermostat Lost Communication to Smart Hub (Meaning, Causes, Fixes)

What the code means

Lennox communicating systems (the iComfort S30 and S40 smart thermostats with their Smart Hub) pass every command over a low-voltage communication bus. On the S30, Lennox says four thermostat wires run between the HD Display and the Smart Hub, and four more run from the Smart Hub to the indoor and outdoor units. Lennox's alert code guide defines alert 124, Tstat Lost Communication To Smarthub, as the thermostat losing communication with a system component for more than three minutes. It's rated Service Urgent. Lennox notes that this alert stops all associated system operations and waits for a heartbeat message from the component that isn't communicating. Lennox's troubleshooting steps are to check the wiring connections between components, ohm the wires (check them for continuity), and cycle power. It warns that any miswired component can cause a false component code to show on another component, so the guide says to disconnect the wiring to the other system components and check communication one at a time. When a condensate float switch is installed, Lennox says to use an isolation relay to break the common wire to the outdoor unit, and for testing, to remove the float switch from the circuit. The alert clears automatically once communication is re-established. The same guide's wiring section says unused wires in a cable with more than four conductors must be bundled and tied to C at the indoor unit rather than left loose, and that communication wiring should be kept about 15 feet from house wiring and large appliances to avoid electrical noise. Related alerts are 31 (lost communication with a known device), 105 (a communication problem often caused by wiring or noise) and 120 (a slow-responding device).

Likely causes

• A loose, corroded or broken low-voltage wire (R, I+, I-, C, or the A and B wires between the HD Display and Smart Hub) somewhere in the communication bus
• A component that has lost power, such as a tripped breaker, an indoor service switch turned off or a blown control fuse
• A condensate float switch that tripped on a clogged drain line, or one wired in a way that interferes with communication (Lennox says to break the common to the outdoor unit through an isolation relay)
• A miswired component, which Lennox says can make a false code show up on a different component
• Unused conductors in a multi-wire thermostat cable left loose instead of capped and tied to C, or communication wiring run too close to house wiring, which lets electrical noise in
• A failing control board or Smart Hub, once wiring and power have been ruled out

Safe DIY checks

1. Open the alert on the thermostat and note which component it names, if any, and whether heating or cooling has stopped.
2. Make sure every unit has power: breakers on, the furnace or air handler service switch on and the outdoor disconnect in place.
3. Check the indoor unit's drain pan and condensate float switch. If water is sitting in the pan, clear the drain line so the switch resets.
4. With power off, look over the low-voltage wiring at the Smart Hub, the thermostat base and the indoor and outdoor control boards for loose, pinched, chewed or corroded wires.
5. Cycle power to the whole system, as Lennox recommends: turn it off at the breakers for a minute, turn it back on and give the thermostat several minutes to reconnect. The alert clears on its own once the component answers.
6. If you're comfortable with a multimeter, Lennox's next step is to ohm each wire for continuity with the power off.
7. If the cable has more wires than are used, check that the spares are capped and tied to C at the indoor unit instead of left loose.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 11 Lennox HVAC System 11
• 292 Lennox HVAC System 292
• 31 Lennox HVAC System 31
• 413 Lennox HVAC System 413
• 433 Lennox HVAC System 433

All Lennox codes →

Sources

1. Lennox Smart Thermostat Basic Communication System Wiring and Alert Codes (100017), Table 2: alert 124 Tstat Lost Communication To Smarthub (Service Urgent, lost communication with a component for more than three minutes, stops associated operations, check connections, ohm wires, cycle power, miswired component can show a false code, isolate components one at a time, float switch isolation relay note, clears automatically); S30 four-wire and unused-wire guidance; alerts 105, 120 and 125
2. Lennox PureAir S (PCO3S) instruction manual alert table reproduced on Manuals+ (same alert 124 Tstat Lost Communication To Smarthub, Service Urgent, wording and steps)