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Lennox · Heat pump

39 No cooling when requested

High severity

What the control is telling you

Cooling could not be turned on for more than 45 minutes; the system goes off-line for 60 minutes, then tries to restart. S30/S40 thermostat alert code 39.

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

Likely causes

  • Outdoor unit fault or lockout
  • Wiring or communication problem

Test and repair steps

  1. Run diagnostics and compare with what the equipment actually does (S30: Dealer Control Center > Diagnostics > Start Diagnostics; S40: Service Support Center > Diagnostics > Select All > Start).
  2. Look for other alert codes that block cooling.
  3. Check all cooling equipment for the cause.
  4. Cycle power. Clears automatically once fixed.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.12. 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.