HVACopedia

Lennox · Furnace

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 gas furnaces (e.g. SL280UHV/SL280UHNV, EL296UHV) 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.

Settings for these units

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

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.

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.