HVACopedia

Lennox · Air conditioner

E402 System pressure trip

Medium severity

What the control is telling you

Compressor overload, or suction or discharge pressure outside its limits. S30/S40 alert code 402; unit control shows E402.

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

Likely causes

  • High or low refrigerant charge
  • Dirty coil or failed fan
  • Restriction

Test and repair steps

  1. Check discharge and suction pressures.
  2. Check outdoor coil, fan and indoor airflow.
  3. Clears after four consecutive normal compressor run cycles.

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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.