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Lennox · Heat pump

E427 Inverter DC peak fault (inverter code 21)

High severity

What the control is telling you

DC peak current of 55 A or higher detected; compressor and fan stop. 10 times in an hour = lockout. Inverter LED flash code 21 (red 2 flashes, green 1 flash). S30/S40 thermostat alert code 427 (shown as E427 on the unit control display).

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

Likely causes

  • Compressor current spike at restart
  • Older inverter software (XP20/XP25 at defrost entry/exit)

Test and repair steps

  1. XP20/XP25 built before serial number 5817F with frequent 427: compare the inverter part number with the latest in repair parts and replace if needed (newer software has current slope logic).
  2. Serial format PPYYMNNNNN YY and M are year and month built.
  3. Refer to the unit service manual.
  4. Clear by disconnecting and reconnecting power to the outdoor control.
  5. Check inverter LEDs flash code 21 = red 2 flashes, green 1 flash. 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.33. 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.