Lennox Furnace Error Code LED#1 Alternating Slow Flash / LED#2 Alternating Slow Flash

This LED pattern indicates a Watchguard lockout caused by repeated burner ignition failure. The furnace attempted to light the burners multiple times without success and shut down as a safety measure. The most common causes include a dirty flame sensor, a failed ignitor, or a gas supply problem. A technician should inspect the gas valve, ignitor, and flame sensor to diagnose the root cause.

The furnace will not produce heat while in Watchguard lockout, so there is no immediate danger from the lockout itself. The safety system is functioning as intended by preventing repeated failed ignition attempts. However, the underlying ignition problem should be diagnosed and repaired promptly, especially during cold weather, to restore reliable heating.

Start by confirming that the gas supply to the furnace is on and that other gas appliances in the home are working. Next, check whether the ignitor glows during the startup sequence by observing through the sight glass if one is present. If the ignitor glows but the burners do not light, the flame sensor or gas valve is the likely issue. Cleaning the flame sensor rod is a straightforward first step before moving on to component testing.

Resetting the furnace by cycling the power or thermostat may temporarily clear the Watchguard lockout and allow the furnace to attempt ignition again. However, if the underlying cause such as a failed ignitor or dirty flame sensor is not corrected, the furnace will fail to ignite again and the fault code will return. A reset is useful for confirming the fault is active but is not a permanent fix without addressing the root cause.

Repair costs depend on which component has failed. Flame sensor cleaning is typically a low-cost service call repair, while replacing a hot surface ignitor generally costs in the range of $150 to $350 including parts and labor. A gas valve replacement is more involved and can range from $300 to $600 or more depending on the part and local labor rates. A full diagnostic visit is the best way to get an accurate estimate.