Lennox Furnace Error Code LED#1 ON / LED#2 ON

Both LEDs being simultaneously illuminated on a Lennox furnace indicates either a control board failure or an incorrect wiring connection at the control board. This is a board-level diagnostic fault, meaning the integrated furnace control has detected a condition it cannot resolve through normal operation. The furnace will not operate normally while this fault is active. A technician should inspect all wiring connections first, then evaluate the board itself for failure.

If the cause is a wiring error, a knowledgeable DIYer comfortable working with low-voltage furnace wiring can compare the installed wiring against the unit's wiring diagram and correct any miswired terminals. However, if the control board itself has failed, replacing the IFC board requires proper identification of the correct replacement part and careful reinstallation of all wiring harnesses. If you are not confident working inside a furnace control panel, a licensed HVAC technician should perform this repair.

If the issue is a wiring correction only, labor costs are typically in the range of $75 to $175 depending on your market and the technician's time. If the integrated furnace control board requires replacement, parts and labor combined can range from $300 to $650 or more, depending on the specific Lennox model, part availability, and regional labor rates. Getting a diagnosis confirmation before committing to a board replacement is advisable, since wiring errors are a less costly resolution.

Cycling power to the furnace may temporarily clear the displayed fault, but if the underlying cause is a failed control board or miswired connection, the fault will return immediately or shortly after restart. A power reset is a reasonable first step, but it is not a fix. If the dual-LED condition returns after reset, proceed with wiring inspection and board evaluation.

The furnace will generally not operate in its normal heating sequence while this fault is active, so the system is effectively locked out rather than running unsafely. You should not attempt to bypass the control board or force operation. Leave the furnace powered off or in lockout until the fault is properly diagnosed and repaired.