Lennox Furnace Error Code E 125

E 125 means the furnace control board has detected an internal hardware failure during its self-check routine. This can include faults in the flame sense circuit, shorted pins, or other internal hardware problems. It is a critical alert that may prevent the furnace from running. The fault clears 300 seconds after the condition recovers, but persistent faults require board replacement.

E 125 is classified as a critical alert, and the furnace may not operate at all when this fault is active. You should not attempt to bypass or ignore this error. If the furnace is not heating, do not use space heaters as a long-term substitute in cold conditions. Contact a qualified technician to diagnose and resolve the fault before relying on the furnace.

Control board replacement involves working with line voltage electrical connections and gas appliance controls, which carries serious safety risks if done incorrectly. While a knowledgeable DIYer may be able to perform this repair, it is strongly recommended that a licensed HVAC technician handle control board diagnosis and replacement to ensure the furnace is correctly configured and safe to operate after the repair.

Cycling power is the recommended first step and may clear the fault if it was caused by a one-time event such as a power fluctuation. However, if the control board has a genuine internal hardware failure, the error will return after the reset. A persistent E 125 after power cycling indicates the board needs to be replaced.

If a power cycle does not resolve the fault, the control board will typically need to be replaced. Control board replacement on a Lennox furnace generally costs between $300 and $700 including parts and labor, depending on the specific model and local labor rates. Diagnostic service call fees may apply in addition to repair costs.