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Lennox · Air conditioner

34 Must program unit capacity for device

High severity

What the control is telling you

The indoor or outdoor control has no programmed unit capacity (tonnage/unit size). S30/S40 thermostat alert code 34.

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

Likely causes

  • Replacement control not programmed with unit size code

Test and repair steps

  1. Remove thermostat power before programming the unit control.
  2. Program the unit capacity at the unit control per its installation instructions (unit size code).
  3. Reconnect the thermostat wires.
  4. S30 go to Menu > Settings > Advanced Settings, open View Dealer Control Center, then pick Equipment > Reset All Equipment. The thermostat will find every Lennox communicating device again.
  5. S40 go to Menu > Settings > Advanced Settings, open View Service Support Center, then Equipment Setting > Reset and pick Reset HVAC Equipment.
  6. Clears automatically once fixed.

Settings for these units

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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.