Lennox Furnace Error Code 11

Error code 11 on a Lennox furnace means the pressure switch was detected in the closed position before the combustion air inducer motor was activated. This is abnormal because the pressure switch should only close after the inducer is running and creating airflow. The furnace holds its startup sequence and will not ignite until the fault is resolved. The most common causes are a stuck pressure switch or a blocked pressure switch hose.

The furnace will not operate while this fault is active, so there is no immediate combustion or carbon monoxide hazard from the fault itself. The pressure switch is a safety component that helps verify proper venting airflow, so it is important to resolve the underlying cause rather than bypass the switch. Do not attempt to bypass or jumper the pressure switch to force operation.

If the fix is simply clearing a blocked or kinked hose, the repair may cost little to nothing beyond a service call fee, which typically ranges from $75 to $150. Replacing a failed pressure switch generally costs between $150 and $350 including parts and labor. If the control board is determined to be faulty, repair costs can range from $400 to $700 or more depending on the model and labor rates in your area.

Start by locating the pressure switch and inspecting the rubber hose that connects it to the inducer housing. Look for kinks, cracks, water inside the hose, or loose fittings, and correct any issues found. If the hose looks good, disconnect it from the switch and use a multimeter to check whether the switch contacts are stuck closed with no pressure applied. A switch that shows continuity without pressure must be replaced.

Turning the thermostat off and back on will reset the startup sequence and allow the furnace to attempt operation again. However, if the pressure switch or hose fault still exists, the error code will return immediately on the next startup attempt. A reset only clears the code temporarily: the root cause must be corrected for the furnace to run normally.