Lennox Furnace Error Code DS1 Fast Flash / DS2 Slow Flash

This error code indicates a problem detected during the ignitor warm-up period. The three possible causes are an improper main ground connection, a broken or open ignitor circuit, or incoming line voltage that has fallen below 75 volts. The furnace control board monitors the ignitor during warm-up and triggers this specific LED flash pattern when one of those conditions is present. Diagnosis involves checking the ground wiring, measuring ignitor resistance, and verifying line voltage at the unit.

When this error is active, the furnace will not complete a successful ignition sequence, so the burners will not light and the unit will go into lockout. The furnace is generally not creating an immediate safety hazard in lockout mode, but you will not have heat. Do not attempt to bypass the fault or force ignition. Address the underlying cause before restoring normal operation.

Resetting the furnace by cycling power may temporarily clear the fault code and allow the unit to attempt ignition again. However, if the underlying problem, whether a failed ignitor, open circuit, wiring fault, or low voltage, has not been corrected, the error will return during the next ignitor warm-up attempt. A reset is useful for confirming the fault is repeatable, but it is not a fix. The root cause must be diagnosed and repaired to resolve the code permanently.

Some parts of the diagnosis, such as inspecting wiring connections and verifying the main ground, can be done by a knowledgeable homeowner with basic electrical knowledge. However, measuring live line voltage and replacing the hot surface ignitor involve working near high-voltage components, which carries real safety risk. If you are not comfortable with electrical diagnostics, a licensed HVAC technician should perform the work. Ignitor replacement itself is straightforward once the fault is confirmed, but the electrical checks require caution.

Repair costs depend on which of the three causes is responsible. A wiring or grounding repair may range from roughly 75 to 200 dollars in labor if no parts are needed. Hot surface ignitor replacement typically runs between 150 and 350 dollars including parts and labor. If low line voltage points to an electrical supply issue requiring an electrician, costs can vary widely depending on the scope of the wiring problem.