HVACopedia

Lennox · Heat pump

65539 Thermostat lost connection or internal fault

High severity

What the control is telling you

System has lost the HD wall display for over 30 seconds. S30/S40 alert code 65539.

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

Likely causes

  • Display off the mag-mount
  • Wiring between hub and display (A/B, +12V/-12V)

Test and repair steps

  1. Check the display is seated and the hub-to-display wiring.
  2. Clears once detected for two seconds.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.47. 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.