Lennox Furnace Error Code LED1-SLOW_FLASH/LED2-OFF

This code means the furnace control board has detected a flame sensor signal while the gas valve has not been commanded to open. The furnace shuts down as a safety precaution because a flame should only be present when the board has specifically energized the gas valve. The fault points to a problem in the flame sensing circuit, the control board, the wiring, or in rare cases, the gas valve itself.

A power reset may temporarily clear the lockout and allow the furnace to attempt another cycle, but if the underlying fault is still present, the code will return quickly. Resetting without diagnosing and repairing the root cause is not a long-term solution and could mask a safety-relevant problem. Persistent or recurring appearances of this code require component-level diagnosis.

Repair costs depend on which component is at fault. Flame sensor replacement is typically the least expensive repair, often in the range of $75 to $200 including labor. Control board replacement is more costly, generally ranging from $300 to $600 or more depending on the board. Gas valve replacement can range from $250 to $600 or higher. A diagnostic service call to identify the root cause usually runs $80 to $150 before parts.

No, you should not attempt to bypass this fault or force the furnace to run. This code indicates a disagreement between what the control board commanded and what the sensors are detecting, which could point to an uncontrolled combustion condition. If a leaking gas valve is the cause, continued operation poses a fire or carbon monoxide risk. Have the system inspected by a qualified technician before restoring operation.

Start by inspecting and cleaning the flame sensor rod, then check the sensor wiring for shorts or damage. Test the flame sensor circuit for leakage current with the gas valve de-energized. If the sensor and wiring are intact, the control board or the gas valve may need to be replaced. A qualified technician with a multimeter and combustion analysis tools can isolate the defective component.