Lennox Furnace Error Code DS1 OFF / DS2 Fast Flash

This error code indicates a problem with the high pressure switch circuit. The furnace has detected that the pressure switch is either open when it should be closed during inducer operation, or closed before the inducer has started. Both conditions signal an abnormal draft pressure situation, and the furnace shuts down as a safety measure. The most common causes are a blocked vent pipe, a faulty pressure switch, or a disconnected pressure switch hose.

Yes, this error is safety-related. The high pressure switch is designed to prevent the furnace from firing without confirmed adequate airflow, which helps ensure combustion gases are properly exhausted. Operating a furnace with a blocked vent or a bypassed pressure switch can create a risk of carbon monoxide buildup in the home. Do not attempt to bypass the pressure switch to get the furnace running.

Resetting the furnace may temporarily clear the fault and allow the furnace to attempt another startup cycle. However, if the underlying cause has not been resolved, such as a still-blocked vent or a failed pressure switch, the error will return and the furnace will shut down again. A reset is useful for confirming whether the fault is intermittent, but it does not substitute for proper diagnosis and repair.

Some causes are DIY-friendly. Clearing a blocked exterior vent termination, reattaching a loose pressure switch hose, or unclogging a condensate drain can often be done without professional tools. However, testing and replacing a failed pressure switch or diagnosing an inducer motor problem requires a multimeter and some comfort working inside the furnace cabinet. If you are unsure, contacting a licensed HVAC technician is the safest option.

Repair costs vary depending on the root cause. If the fix is simply clearing a blocked vent, costs are minimal, typically a service call fee in the range of $75 to $150. Replacing a pressure switch usually costs between $100 and $250 including parts and labor. Replacing a failed inducer motor is more involved and can range from $300 to $700 or more depending on the specific model and local labor rates.