Lennox Furnace Error Code DS1 Alternating Slow Flash / DS2 Alternating Slow Flash

This alternating slow flash pattern between DS1 and DS2 indicates a Watchguard lockout condition on the Lennox furnace control board. It activates when the burners fail to ignite, when a limit switch stays open for more than 3 minutes, when flame sense is lost 5 times in one heating cycle, or when a pressure switch opens 5 times in one cycle. The furnace will not attempt to restart until the fault is cleared. Diagnosing the specific trigger requires checking the gas supply, ignitor, flame sensor, limit switches, and pressure switches.

The Watchguard lockout is a built-in safety function, so the furnace has already shut itself down and is not operating while in lockout. It is safe to investigate the cause, but you should not attempt to bypass the safety controls or force repeated manual resets without identifying the problem. If the fault is related to a limit switch staying open, this can indicate the heat exchanger is overheating, which should be addressed promptly to avoid damage to the furnace.

Start by confirming the gas supply is on and the shutoff valve is fully open, then check the air filter and make sure all supply and return vents are unobstructed. Next, inspect the flame sensor rod for oxidation or buildup and check visible pressure switch hoses for cracks or disconnections. If the basic checks do not reveal an obvious problem, a technician with diagnostic tools can measure ignitor resistance, check pressure switch operation with a manometer, and evaluate blower motor performance to pinpoint the cause.

You can reset the furnace by cycling the power off and back on, which will clear the Watchguard lockout and allow the furnace to attempt another ignition sequence. However, if the underlying problem is not corrected first, the furnace will go through its ignition trials again, fail under the same conditions, and return to lockout. Resetting without diagnosing and repairing the root cause is not a lasting solution and repeated lockouts can indicate a progressively worsening component failure.

Repair costs vary significantly depending on the root cause. Cleaning a flame sensor is a low-cost repair, often falling in the range of $80 to $150 for a service call if done by a technician. Replacing a hot surface ignitor typically costs between $150 and $300 including parts and labor. More extensive repairs such as a failed draft inducer motor or a damaged pressure switch assembly can range from $250 to $600 or more depending on the specific components and local labor rates.