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Lennox · Air handler

E347 Indoor or EIM relay Y1 fault

High severity

What the control is telling you

The indoor control did not see 24 VAC between Y1 and C after relaying a Y1 compressor demand; operation stops. S30/S40 thermostat alert code 347 (shown as E347 on the unit control display).

Applies to: Lennox communicating air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems

Likely causes

  • Y1 relay failed (contacts did not close or coil did not energize)

Test and repair steps

  1. Measure 24 VAC between Y1 and C at the indoor control during a cooling call.
  2. If no voltage, the Y1 relay / stage 1 output has failed; replace the control.
  3. Clears after reset and Y1 input sensed.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.26. 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.