Lennox Furnace Error Code LED1 ALT SLOW FLASH / LED2 ALT SLOW FLASH

This alternating slow flash pattern on both diagnostic LEDs indicates a Watchguard lockout condition. The furnace control board triggers this fault when the burners fail to ignite, when a limit switch stays open for more than 3 minutes, or when the flame sensor loses the flame signal 5 times in one heating cycle. The Watchguard function exists to prevent unsafe repeated ignition attempts when a core problem is detected.

Yes, this code reflects a safety lockout that the furnace initiated on its own to prevent a potentially hazardous condition. A furnace that cannot confirm ignition or that is overheating due to restricted airflow should not be forced to run. Do not attempt to bypass the lockout or disable the Watchguard function. Have the system inspected by a qualified technician before returning it to normal operation.

Repair costs vary significantly depending on the root cause. Cleaning a flame sensor is a low-cost service call, typically in the range of $80 to $150 for labor. Replacing a hot surface ignitor generally costs between $150 and $300 including parts and labor. A gas valve replacement or control board issue can bring costs to a range of $350 to $600 or higher. Getting a diagnostic visit to confirm the cause before authorizing repairs is always recommended.

Resetting the furnace by cycling power may temporarily clear the lockout and allow the furnace to attempt ignition again. However, if the underlying cause is not corrected, the Watchguard fault will return after the same failure conditions are met. A reset is useful for confirming the fault is active, but it is not a fix and should be followed by a thorough inspection of the gas supply, ignitor, and flame sensor.

Start by checking that the gas supply valve is fully open and that other gas appliances in the home are working, which rules out a broader supply issue. Next, inspect the air filter and replace it if clogged, then check the flame sensor rod for buildup that can be cleaned with fine steel wool. If the ignitor is cracked or the gas valve wiring is damaged, those components will need repair or replacement. If these steps do not resolve the fault, a technician should perform a complete ignition sequence inspection.