HVACopediaLennox · Air handler
E126 Control internal communication problem
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 air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems
Likely causes
- Control internal fault
Test and repair steps
- The control usually resets itself; cycle power.
- Replace the control if the problem is persistent and stops operation.
- Clears 300 seconds after the issue ends.
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.
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.