HVACopediaLennox · Air handler
551 Zone sensor lost communication
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 air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems
Likely causes
- Loose, damaged or incorrect wiring between damper module and sensor
Test and repair steps
- Check wiring between the damper control module and the zone sensor.
- 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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