Lennox · Air handler

20 Protocol upgrade required

High severity

What the control is telling you

Thermostat firmware is too old for one connected device. Update the firmware so they can talk. S30/S40 alert code 20.

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

Likely causes

Thermostat firmware older than a connected device

Test and repair steps

Update the thermostat firmware (Wi-Fi connection / auto update).
Re-check that all devices are detected after the update.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.9. 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.