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Lennox · Heat pump

E401 Compressor long run cycle

Low severity

What the control is telling you

The compressor ran more than 18 hours continuously on one cooling demand, or system pressure is low. Also monitors low pressure switch trips. Does not lock out. S30/S40 thermostat alert code 401 (shown as E401 on the unit control display).

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

Likely causes

  • Low refrigerant charge
  • Undersized system or extreme load
  • Low pressure switch trips

Test and repair steps

  1. Check refrigerant charge and pressures.
  2. Two-stage units with LED outdoor control run in low speed; units with seven-segment display show 401 but keep running high speed.
  3. Heat pumps ignore 401 when outdoor temperature is below 65 F.
  4. Goes away after a power reset, or after 30 normal run cycles in a row.

Settings for these units

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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.

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.