Lennox E125 Error Code

What does Lennox E125 mean?

E125 indicates a hardware problem on a communicating system component's control board. The component has detected an internal fault in its own hardware during self-check. The system places the affected component in a reduced or non-operational state. The alert clears five minutes after the issue no longer exists. In zoning systems, all dampers remain open (non-zone mode) for five minutes after recovery.

Symptoms

• E125 or 'Control Hardware Problem' displayed on the thermostat.
• The affected component stops responding normally to system demands.
• Zone systems revert to all-dampers-open mode while E125 is active.
• Alert may be intermittent if the hardware fault is temperature- or vibration-dependent.

Common causes

• Failed or failing control board on the indoor unit, outdoor unit, or communicating accessory.
• Missing jumper on a PureAir S control board.
• R-to-W2 jumper present on an indoor unit connected to an Equipment Interface Module (EIM) — this jumper must be removed.
• Internal self-check failure — firmware or hardware corruption on the control.

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Diagnostic steps

1. Identify the component reporting E125 from the thermostat alert text

   The thermostat identifies which component's hardware has failed. Navigate to the Notifications or Alerts section of the dealer control center to confirm which device is flagging E125.

2. Check for a jumper on R to W2 if an EIM is in the system

   Per Lennox documentation, if an Equipment Interface Module is installed, remove any jumper present between R and W2 on the indoor unit terminal strip. This jumper causes E125 in EIM configurations.

3. Inspect the PureAir S control board for a missing selector jumper

   If a PureAir S is in the system and is the reporting component, check that the jumper selector on the PureAir S control board is present and correctly positioned. A missing jumper triggers E125 on the accessory.

4. Cycle power to the affected component

   Turn off the breaker serving the affected component for 30 seconds, then restore power. A transient hardware fault clears with a power cycle. If E125 returns immediately, the control board requires replacement.

5. Replace the control board if E125 is persistent

   If none of the above steps resolve E125 and the condition prevents normal operation, replace the control board on the affected component with the Lennox-specified part for the model.

When to call a professional

Control board replacement on communicating furnaces, air handlers, and outdoor units requires sourcing the correct part for the communicating platform, reconfiguring unit size codes after replacement, and recommissioning the full system. This should be performed by a licensed Lennox dealer with access to the installer configuration menus.