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Lennox · Heat pump

700 Thermostat temperature sensor problem

High severity

What the control is telling you

Built-in temp sensor in the HD display is faulty. S30/S40 alert code 700.

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

Likely causes

  • Display not seated on mag-mount
  • Drafts from the wall cavity
  • Failed display

Test and repair steps

  1. Remove the HD display from the mag-mount and re-attach it.
  2. Seal the hole in the wall behind the mag-mount.
  3. Run Reset All in the dealer control center.
  4. Replace the HD display if it persists.

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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.