Lennox Furnace Error Code DS1 OFF / DS2 Slow Flash

This error code indicates a problem with the low pressure switch circuit. The fault appears when the low pressure switch is open during inducer operation, when the inlet or exhaust vent is blocked, or when the pressure switch is detected as closed before the inducer motor starts. The furnace will not proceed to ignition until the pressure switch circuit confirms proper draft is established.

The furnace will not allow ignition to proceed while this fault is active, which is by design. The pressure switch is a safety control that confirms proper venting before combustion begins. You should not attempt to bypass or jumper the pressure switch, as doing so removes a critical safeguard against combustion gas accumulation in the living space.

Repair costs vary depending on the cause. Clearing a blocked vent yourself costs nothing beyond your time, while a service call for professional diagnosis typically runs between $75 and $150. Replacing a pressure switch generally costs between $100 and $250 including parts and labor. Inducer motor replacement, if needed, is a more involved repair that can range from $300 to $600 or more depending on the motor and labor rates in your area.

Start by inspecting the exterior vent terminations for ice, debris, or obstructions and clear any blockages you find. Next, check the pressure switch hose for cracks or loose connections. If the venting is clear and the hose is intact, use a multimeter to test the pressure switch for continuity at rest and during inducer operation. A switch that does not respond correctly to inducer draft will need to be replaced.

A reset may temporarily clear the error and allow the furnace to attempt another startup cycle, but the fault will return if the underlying cause is not corrected. If the vent blockage or pressure switch fault is still present, the control board will detect the same condition and display the error again. Resolving the root cause is necessary before the error will stay cleared.