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Lennox · Heat pump

E424 Liquid line sensor faulty

Medium severity

What the control is telling you

The liquid line temperature sensor lost a valid reading. S30/S40 thermostat alert code 424 (shown as E424 on the unit control display).

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

Likely causes

  • Open/shorted sensor or wiring

Test and repair steps

  1. Compare sensor resistance with the chart in the unit installation instructions.
  2. Replace the sensor pack if needed.
  3. Sensors are detected at configuration; clears on configuration or normal readings.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.33. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

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.