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Lennox · Furnace

551 Zone sensor lost communication

Medium severity

What the control is telling you

A zone sensor lost communication with the damper control module; zone shows '--' and the system runs central mode. S30/S40 thermostat alert code 551.

Applies to: Lennox communicating gas furnaces (e.g. SL280UHV/SL280UHNV, EL296UHV) in S30/S40 smart thermostat systems

Likely causes

  • Loose, damaged or incorrect wiring between damper module and sensor

Test and repair steps

  1. Check wiring between the damper control module and the zone sensor.
  2. Clears and zoning returns when communication is restored.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.46. 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.