Lennox · Air conditioner

E411 Low pressure switch strikes lockout

High severity

What the control is telling you

Low pressure switch opened five times in one cooling or heating demand; the outdoor unit is shut down. S30/S40 thermostat alert code 411 (shown as E411 on the unit control display).

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

Likely causes

Low refrigerant charge
Low indoor airflow
Restriction or failed switch

Test and repair steps

Check system charge using both approach and subcooling methods.
Check airflow and look for restrictions.
Reset by putting the outdoor control in test mode or cycling low-voltage power.

Settings for these units

Outdoor unit clearances · Lennox EL16KC1

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.30. 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.