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Lennox · Air conditioner

E423 Inverter CT circuit fault (inverter code 40)

High severity

What the control is telling you

The inverter detected a current transformer circuit issue; outdoor unit stops and the anti-short-cycle timer starts. Locks out after 10 strikes in one hour. Inverter LED flash code 40 (red 4 flashes, green off). S30/S40 thermostat alert code 423 (shown as E423 on the unit control display).

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

Likely causes

  • Inverter current sensing circuit fault

Test and repair steps

  1. Refer to the outdoor unit service manual for inverter troubleshooting.
  2. Service urgent disconnect power to the outdoor unit and restart to clear.
  3. Check inverter LEDs flash code 40 = red 4 flashes, green off. Normal inverter LEDs with no fault: red ON, green OFF.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.32. 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.