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

This LED pattern means the furnace control board is detecting a flame signal even though the gas valve has not been energized. It is a safety fault because sensing a flame outside of a normal ignition sequence indicates either a false signal from the sensor circuit or, more seriously, gas flowing without board command. The furnace will lock out to prevent unsafe operation until the fault is corrected.

Start by checking the gas valve to confirm it is not passing gas when the board has not sent a 24-volt signal to open it. If the valve is functioning correctly, move to the flame sensor: inspect the rod for carbon deposits or a cracked insulator and examine the sensor wiring for shorts. Cleaning or replacing the flame sensor resolves this fault in many cases, but a defective control board may also need to be considered if the other components test out properly.

Repair costs vary depending on which component is at fault. Flame sensor replacement is generally the least expensive fix, typically in the range of $75 to $200 including labor. A gas valve replacement is more involved and can range from $200 to $600 or more depending on the model and labor rates in your area. Control board replacement tends to fall in a similar or higher range, so accurate diagnosis before ordering parts is important to avoid unnecessary expense.

You should not continue operating the furnace with this fault active. If the gas valve is sticking open and allowing uncontrolled gas flow, running the system could create a hazardous condition. Turn off the furnace and have a qualified technician inspect the gas valve and flame sensor circuit before attempting to restore normal operation.

A power reset may temporarily clear the fault and allow the furnace to attempt another startup cycle, but the error will return if the underlying problem is not corrected. Because this fault involves a potentially unsafe condition, resetting without diagnosing the root cause is not a recommended long-term approach.