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

This code means the furnace detected a problem during the ignitor warm-up period, which is the stage just before the gas valve opens. The board is flagging one of three possible issues: an improper main ground, a broken or open ignitor circuit, or incoming line voltage below 75 volts AC. The alternating fast flash pattern on both diagnostic LEDs is specific to this warm-up stage fault, which helps distinguish it from faults that occur later in the ignition sequence.

Start by checking the three items the code points to: the main ground connection, the ignitor resistance, and the incoming line voltage. Use a multimeter to test ignitor resistance and look for an open circuit reading, which means the ignitor needs replacement. Verify the ground wire is securely attached at the furnace chassis, and confirm line voltage at the furnace is at least 75 volts AC while the unit is calling for heat. If voltage is low, an electrician may need to evaluate the supply circuit.

If the ignitor is the cause, replacement parts and labor typically range from $150 to $350 depending on the specific model and local labor rates. Wiring or ground connection repairs tend to fall on the lower end of that range since no parts are usually required. If low line voltage is the culprit, electrical panel or circuit work by a licensed electrician could add additional cost. Getting a diagnostic visit from a technician will clarify exactly which component is at fault before committing to a repair cost.

Resetting the furnace by cycling the power may temporarily clear the code and allow another ignition attempt, but the error will return if the underlying problem is not corrected. A failed ignitor, bad ground, or low voltage condition will cause the fault to repeat each time the furnace tries to light. Resetting is useful for confirming the fault is consistent, but it is not a fix.

The furnace will not produce heat while this fault is active because the ignitor is not functioning correctly, which also means no gas is being released. This makes it a non-heating failure rather than a safety emergency in most cases. However, the furnace should not be left to cycle repeatedly without addressing the cause, as persistent retry attempts can stress other components. Contact a qualified technician to diagnose and correct the fault before relying on the furnace for heat.