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Lennox · Air conditioner

30 Low temperature protection (below 40 F)

High severity

What the control is telling you

Cooling is blocked until the thermostat reads above 40 F. S30/S40 thermostat alert code 30.

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

Likely causes

  • Cooling equipment stuck on
  • Thermostat sensor reading wrong

Test and repair steps

  1. Check that cooling equipment is not stuck ON (contactor, compressor).
  2. Verify the thermostat's temperature sensor reads correctly.
  3. Use heating mode to warm the space above 40 F.
  4. Clears automatically once fixed.

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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.