HVACopediaLennox · Air handler
31 Lost communication with device
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 handlers and EIM-equipped indoor 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
- Check connections and voltages at the device.
- Ohm the wires for continuity.
- If a float switch is on the air handler drain pan, check for a clogged condensate line tripping it.
- Check whether a freezestat is installed and open.
- 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.
Homeowner looking at this code? See what it means for you and when to call a technician.
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.