Lennox Furnace Error Code LED1-ALT_SLOW_FLASH/LED2-ALT_SLOW_FLASH

This code indicates a Watchguard fault, meaning the furnace control board detected that the burners failed to ignite after multiple attempts. The alternating slow flash pattern on both LED indicators is the board's way of signaling this specific lockout condition. The furnace will not attempt to heat again until the fault is addressed and the system is reset.

Start by confirming that gas is being supplied to the furnace: check that the manual shutoff valve is fully open and verify line pressure at the gas valve inlet. If gas supply is confirmed, the next steps involve inspecting the igniter, testing the gas valve for proper operation, and cleaning or testing the flame sensor. A qualified technician should perform voltage and resistance testing on these components to identify the specific failure.

Repair costs vary depending on which component has failed. Flame sensor cleaning or igniter replacement is typically in the range of $100 to $300 including labor. Gas valve replacement tends to cost more, often in the range of $250 to $600 depending on the part and labor rates in your area. Control board replacement is generally the most expensive repair, often ranging from $400 to $700 or more.

Resetting the furnace by cycling power may clear the lockout temporarily and allow the furnace to attempt ignition again. However, if the underlying cause is not corrected, the burners will fail to ignite again and the Watchguard fault will return. A reset is useful for confirming the fault is active, but it is not a fix on its own.

The furnace will not continue operating while this Watchguard lockout is active, so there is no ongoing risk from the furnace itself continuing to run. However, you should not repeatedly reset and attempt to force ignition without diagnosing the cause, as repeated failed ignition attempts can allow unburned gas to accumulate. If you smell gas at any point, leave the home and contact your gas utility immediately.