Lennox SLP98UHV Error Code E 125: Control Self-Check Failed
What Does Code E 125 Mean?
Error code E125 on the Lennox SLP98UHV indicates that the control board has failed its internal self-check. This is a hardware-level error covering a range of internal component failures including flame sense circuit faults, pin shorts, and other electronic malfunctions on the board itself.
The control board runs a self-diagnostic routine that checks its own hardware before and during operation. When a component on the board does not respond correctly — such as a relay not switching, a sensor circuit reading incorrectly, or internal communication between processors failing — the board flags E125. Power surges, lightning strikes, moisture intrusion, and age-related component degradation are common causes of control board hardware failures.
The board will attempt to restart if the error clears on its own, so cycling power to the furnace (off for 30 seconds, then on) is worth trying as a first step. However, if E125 persists or keeps returning, the control board has a permanent hardware defect and needs to be replaced by a qualified technician.
Common Causes
When to Call a Professional
Contact a licensed HVAC technician if:
- E125 returns after cycling power to the furnace
- The furnace is not operating due to the control board failure
- The control board likely needs to be replaced
- E125 appeared after a power surge or lightning event
✓ Verified against manufacturer service manual — March 2026