Lennox Furnace Error Code 9

Error code 9 means the pressure switch failed to close during a call for heat, or it closed and then opened while the furnace was running. The pressure switch monitors the draft created by the combustion air inducer motor, and the furnace will not allow ignition unless safe venting pressure is confirmed. The most common triggers are a blocked pressure switch hose, a restricted flue, or a failing inducer motor. The furnace enters a 5-minute lockout before it will attempt to restart.

The furnace has shut itself down specifically to prevent unsafe operation, so it will not fire while the code is active. Do not attempt to repeatedly reset the unit and force it through cycles without addressing the root cause. A restricted flue or failing inducer could allow combustion gases to back-draft into the living space if venting is compromised, so leave the furnace off until the fault is diagnosed and corrected.

Repair cost varies significantly depending on the root cause. Clearing a blocked flue or pressure switch hose during a service call typically falls in the range of $80 to $200 for the diagnostic visit and labor. Replacing a pressure switch generally runs $100 to $250 including parts and labor. An inducer motor replacement is the most expensive common repair, typically ranging from $300 to $650 or more depending on the specific unit and local labor rates.

Some causes of code 9 are accessible to a knowledgeable homeowner. Checking the exterior flue termination for blockages, inspecting the small rubber hose inside the furnace for obvious cracks or disconnections, and clearing a backed-up condensate drain are all reasonable DIY steps. However, testing the pressure switch with a multimeter, evaluating the inducer motor capacitor, or replacing components requires comfort with electrical diagnostics and should be handled by a qualified technician if you are uncertain.

A manual reset or power cycle will clear the displayed fault and allow the furnace to attempt another ignition sequence after the 5-minute reset period expires. However, if the underlying cause is not corrected, the furnace will detect the same pressure switch failure and lock out again. Repeated resets without repair can sometimes indicate a marginal component that is failing intermittently, and the fault will return with increasing frequency.