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Lennox · Air handler

35 Incorrect operation of device

High severity

What the control is telling you

The unit did not respond within 15 minutes to a thermostat request to start or stop heating/cooling; a device was disabled by a fault/lockout, or electrical noise from the contactor coil disrupted communication. S30/S40 thermostat alert code 35.

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

Likely causes

  • Device in fault/lockout
  • Electrical noise when the compressor contactor coil energizes

Test and repair steps

  1. Check the device's own fault/lockout code and fix that first.
  2. On a communicating system, add a transient voltage suppressor across (parallel to) the coil terminals of the outdoor unit's compressor contactor.
  3. Non-communicating outdoor unit wire suppressor 89W72 across the contactor coil or across Y1 and C at the indoor control.
  4. Suppressor part Littelfuse 5KP43CA bidirectional TVS diode; see Lennox note IAQ-10-01.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.11. 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.