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Lennox · Furnace

E381 EIM interlock relay stuck

High severity

What the control is telling you

EIM interlock relay input sensed continuously with the relay off; no heating or cooling. Escalates after 10 minutes. S30/S40 thermostat alert code 381 (shown as E381 on the unit control display).

Applies to: Lennox communicating gas furnaces (e.g. SL280UHV/SL280UHNV, EL296UHV) in S30/S40 smart thermostat systems

Likely causes

  • Stuck relay contacts
  • External voltage on the input

Test and repair steps

  1. Check for external voltage on the interlock input.
  2. Replace the EIM if the relay is stuck.
  3. Clears 30 seconds after the fault clears.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.28. 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.