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Lennox · Heat pump

E400 Compressor internal overload tripped

Medium severity

What the control is telling you

Y1 demand present but the compressor is not running; its internal overload has tripped. S30/S40 thermostat alert code 400 (shown as E400 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 overheated
  • Loss of power to the unit
  • Low charge, high head pressure

Test and repair steps

  1. Check power to the outdoor unit.
  2. Let the compressor cool and check why the overload tripped (charge, airflow, pressures).
  3. Goes away when the RUN and START sensors both see current for two seconds or more, when service is removed, or on a power reset.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.29. 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.