Lennox Furnace Error Code 3 Flashes

Three flashes on a Lennox furnace with Control 103085 means the pressure switch remained open while the combustion air inducer motor was energized. The pressure switch monitors draft airflow through the heat exchanger, and it must close its contacts when the inducer runs to confirm adequate airflow before ignition is allowed. When the switch stays open, the control board halts the heating cycle as a safety precaution. Common causes include blocked vents, a faulty pressure switch hose, or a failed pressure switch.

Yes, but the error code itself means the furnace is behaving safely by refusing to ignite without confirmed airflow. The pressure switch exists specifically to prevent combustion from occurring if draft is inadequate, which could otherwise lead to carbon monoxide buildup in the living space. You should not attempt to bypass the pressure switch to restore heat. Address the underlying cause before operating the furnace.

A reset may temporarily clear the lockout and allow the furnace to attempt another ignition cycle, but if the underlying problem is not corrected the fault will return and the furnace will lock out again. Repeatedly resetting without diagnosing the cause does not fix the issue and can mask a condition that warrants attention. Diagnose and correct the source of the pressure switch fault before relying on the furnace for heat.

Start by checking the intake and exhaust vent pipes at the exterior of your home for any blockage such as ice, snow, or debris. Next, trace the small rubber hose connecting the inducer housing to the pressure switch and look for cracks, kinks, or loose fittings. If the vents and hoses are clear and secure, the pressure switch itself or the inducer motor may need testing or replacement. A qualified technician can use a manometer to verify whether the inducer is producing adequate negative pressure and whether the switch is opening at the correct setpoint.

Repair costs vary widely depending on the root cause. Clearing a blocked vent or reseating a loose pressure switch hose may cost nothing beyond a service call fee, typically in the range of $75 to $150. Replacing a pressure switch generally runs between $100 and $250 including parts and labor. An inducer motor replacement is the most expensive scenario, often ranging from $300 to $600 or more depending on the specific model and local labor rates.