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Lennox · Air handler

E124 Thermostat lost communication to smart hub

High severity

What the control is telling you

The thermostat has lost communication with a component for more than three minutes; all related operation stops until it hears from that component. S30/S40 thermostat alert code 124 (shown as E124 on the unit control display).

Applies to: Lennox communicating air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems

Likely causes

  • Loose or miswired communication wires
  • A miswired component causing false codes
  • Float switch wiring

Test and repair steps

  1. Check wiring between all components and ohm the wires.
  2. Cycle power.
  3. Disconnect all other components and check communication one device at a time.
  4. Float switch use an isolation relay to break the common to the outdoor unit; remove the float switch for testing.
  5. Clears when communication is re-established.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.17. Summarized in our own words; check the original manual for your model.

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For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.