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Lennox · Air conditioner

E125 Control hardware problem

High severity

What the control is telling you

A hardware problem on a component control. S30/S40 thermostat alert code 125 (shown as E125 on the unit control display).

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

Likely causes

  • Failed control board
  • EIM: jumper left between R and W2 on indoor unit
  • PureAir S: jumper selector missing

Test and repair steps

  1. Zoning system stays in non-zone mode (dampers open) for five minutes after the condition ends.
  2. With an Equipment Interface Module, remove any jumper between R and W2 on the indoor unit.
  3. With PureAir S, install the missing control board jumper selector.
  4. Otherwise replace the control if the problem is persistent and stops operation.
  5. Clears five minutes 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.17. 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.