Lennox Furnace Error Code DIAG#1: Off / DIAG#2: Flash

This code means the furnace flame sensor is detecting a flame signal at a time when the control board has not activated the gas valve. The furnace shuts down as a safety precaution because an unexpected flame indicates either a gas valve that is leaking or stuck open, a wiring fault in the sensor circuit, or a failed flame sensor producing a false reading. This is considered a safety-critical fault and should be diagnosed promptly.

A power reset may temporarily clear the fault and allow the furnace to attempt another cycle, but if the underlying cause is still present, the code will return. Because this fault involves a potential gas valve or flame detection issue, resetting without diagnosing and repairing the root cause is not a safe long-term solution.

Repair costs vary depending on the failed component. A flame sensor replacement is typically a lower-cost repair, often in the range of $75 to $200 including labor. A gas valve replacement is more involved and can range from $300 to $600 or more depending on the model and labor rates. Control board replacement tends to fall in the $400 to $700 range. A technician diagnosis fee will usually apply before parts costs are determined.

No, you should not continue operating the furnace with this fault active. A flame sensed without gas valve activation can indicate uncontrolled combustion inside the unit, which is a fire and carbon monoxide risk. Turn off the furnace at the thermostat and the gas supply, and have a qualified technician inspect the system before restarting it.

Diagnosis should start with inspecting the gas valve to confirm it closes fully when de-energized, then checking the flame sensor wiring for shorts or damage. The flame sensor itself should be tested for proper operation. If the gas valve is leaking or stuck, it must be replaced. Wiring faults can sometimes be repaired, but a failed sensor or control board will require part replacement.