Lennox Furnace Error Code 10

Error code 10 is a Watchguard fault indicating the pressure switch opened five times during a single heating demand. The pressure switch monitors airflow created by the combustion air inducer, and if it cannot stay closed, the furnace shuts down the ignition sequence. After five failures, the furnace enters a one-hour protective lockout. Common causes include a blocked flue, a weak inducer motor, a damaged pressure switch hose, or a faulty pressure switch.

The code itself indicates the furnace safety system is working correctly by shutting down before a hazardous condition develops. The one-hour Watchguard lockout prevents repeated failed ignition attempts that could stress the heat exchanger. However, a blocked flue or venting issue that is left unresolved could pose a risk of carbon monoxide buildup, so the underlying cause should be diagnosed and corrected promptly before the furnace is returned to service.

Repair costs vary depending on the cause. Clearing a blocked vent termination or reattaching a hose may cost little to nothing beyond a service call fee, which typically ranges from $75 to $150. Replacing a pressure switch generally runs $100 to $300 including parts and labor. Replacing a combustion air inducer motor is a more significant repair, often ranging from $300 to $700 or more depending on the specific model and local labor rates.

Some causes of code 10 are accessible to a knowledgeable DIYer, such as clearing a blocked exterior vent termination, reconnecting a loose pressure switch hose, or clearing a clogged condensate drain. However, diagnosing a failing inducer motor, testing pressure switch threshold values, or replacing internal components typically requires a technician with proper tools and training. If you are not comfortable working around gas appliances and electrical components, contact a licensed HVAC technician.

Resetting the furnace by cycling the power or adjusting the thermostat can clear the lockout and allow the furnace to attempt heating again, but it will not resolve the underlying problem. If the root cause such as a blocked vent or faulty component is not corrected, the pressure switch will open again during the next heating cycle and the code will return. Repeated resets without diagnosis can also accelerate wear on the inducer motor and heat exchanger.