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Lennox · Air conditioner

31 Lost communication with device

High severity

What the control is telling you

A component (indoor, outdoor, EIM or damper control module) has not talked to the thermostat for more than three minutes. S30/S40 thermostat alert code 31.

Applies to: Lennox communicating air conditioners incl. variable-capacity inverter units in S30/S40 smart thermostat systems

Likely causes

  • Loose or open communication/24 V wiring
  • Float switch on air handler drain pan tripped (wired in series with R)
  • Freezestat installed and open

Test and repair steps

  1. Check connections and voltages at the device.
  2. Ohm the wires for continuity.
  3. If a float switch is on the air handler drain pan, check for a clogged condensate line tripping it.
  4. Check whether a freezestat is installed and open.
  5. Cycle power if it persists; clears when communication returns.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.10. 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.