Lennox Furnace Error Code DS1 Slow Flash / DS2 Off

This code means the furnace control board is detecting a flame signal even though the gas valve has not been turned on. It is a safety lockout designed to prevent uncontrolled combustion. The most common causes are a contaminated flame sensor rod or a short to ground in the sensor wiring. The furnace will not operate normally until the fault condition is identified and corrected.

Yes, this fault should be taken seriously. The control board has detected a flame signal at a time when no flame should exist, which could indicate uncontrolled gas combustion or a circuit fault. The furnace locks itself out as a protective measure. Continued operation attempts without diagnosing the root cause are not recommended.

Repair costs vary depending on what is causing the fault. Cleaning a contaminated flame sensor is a relatively low-cost service call, typically in the range of $75 to $150 for labor. Replacing a flame sensor itself generally runs $100 to $200 including parts and labor. If the gas valve or control board needs replacement, costs can range from $300 to $700 or more depending on the specific components and local labor rates.

Start by inspecting the flame sensor rod for carbon or oxidation buildup and cleaning it with fine steel wool if contamination is present. Next, trace the flame sensor wire from the sensor to the control board and look for any damaged insulation or contact with the metal cabinet. If the wiring and sensor appear intact, a technician will need to evaluate the gas valve and control board for failure. Do not bypass the lockout, as it exists to protect against a potentially hazardous condition.

A power reset may temporarily clear the fault and allow the furnace to attempt operation again, but if the underlying cause is still present the code will return. A contaminated sensor, shorted wire, or failed component will continue to trigger the fault on every cycle until it is repaired. Resetting without diagnosing is not a long-term solution.