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Lennox · Heat pump

E126 Control internal communication problem

High severity

What the control is telling you

Internal hardware problem on a component control; with zoning it can also trigger when a zone keeps drifting away from set point. S30/S40 thermostat alert code 126 (shown as E126 on the unit control display).

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

Likely causes

  • Control internal fault

Test and repair steps

  1. The control usually resets itself; cycle power.
  2. Replace the control if the problem is persistent and stops operation.
  3. Clears 300 seconds after the issue ends.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.18. 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.