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Lennox · Heat pump

E409 Outdoor control low 24 VAC

Medium severity

What the control is telling you

Secondary voltage at the outdoor control fell below 18 VAC; after 10 minutes the thermostat turns that unit off. S30/S40 thermostat alert code 409 (shown as E409 on the unit control display).

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

Likely causes

  • Overloaded or weak transformer
  • Long or undersized thermostat wire
  • Loose connection

Test and repair steps

  1. Measure 24 VAC at the outdoor control.
  2. Check transformer load and wiring.
  3. Clears after voltage is above 20 VAC for two seconds or 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.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.