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

This LED pattern indicates a low flame signal fault, meaning the flame sense rod is returning a current reading below 0.7 microamps to the control board. The furnace cannot reliably confirm that the burner flame is present, so it shuts down as a safety measure. The most common cause is a dirty or failed flame sense rod.

The furnace is designed to shut down when flame signal drops below the minimum threshold, which is itself a safety response preventing unconfirmed combustion from continuing. The fault code does not indicate a gas leak or immediate danger, but the furnace will not operate normally until the issue is resolved. You should not attempt to bypass the flame sensing circuit.

Resetting the furnace by cycling power may temporarily clear the fault and allow the furnace to attempt ignition again, but the error will return if the underlying cause is not corrected. The flame sense rod will still be dirty, misaligned, or failed, and the measured current will again fall below 0.7 microamps within the same or subsequent heating cycle. A reset is useful for confirming the fault is active but is not a repair.

A knowledgeable DIYer can attempt to clean the flame sense rod using fine steel wool or light abrasive to remove oxidation from the rod tip, which sometimes restores adequate signal current. If cleaning does not resolve the fault, the rod should be replaced, which is a straightforward part swap for someone comfortable working around gas appliances. Any work involving gas connections or the control board should be left to a licensed technician.

If only the flame sense rod needs replacement, parts typically cost between $10 and $40, and a service call with labor usually runs between $80 and $200 depending on your region and the technician's rates. If the control board is the underlying cause, board replacement costs can range from $150 to $600 or more for parts alone, plus labor.