Lennox HVAC Error Code 3 Flashes

A 3-flash code on a Lennox furnace indicates that either the pressure switch circuit is open or closed when it should not be, or the auxiliary limit switch has opened due to high temperature. Both are safety-related faults that cause the system to shut down. The code will not appear on the LED until approximately 120 seconds after the indoor blower has been de-energized. Diagnosing which specific component triggered the fault requires inspecting both the pressure switch circuit and the limit switch circuit.

Yes, this code reflects the furnace protecting itself from potentially unsafe conditions, either inadequate venting or excessive heat. You should not attempt to bypass or disable the safety switches that triggered this fault. Until the underlying cause is identified and corrected, the system will continue to shut down on this fault to prevent heat exchanger damage, carbon monoxide risk, or fire hazard.

Resetting the furnace by cycling power may temporarily clear the fault code and allow the system to attempt another heating cycle, but if the underlying problem is still present, the system will fault again and display the 3-flash code. The 120-second delay before the code appears means the system will run through a portion of the startup sequence before shutting down again. A reset is useful for confirming an intermittent condition but does not fix the cause of the fault.

Some causes are DIY-accessible, such as replacing a clogged air filter, clearing a blocked return vent, or inspecting the vent pipe termination cap for debris or ice. If the filter is clean and the venting appears unobstructed, the fault likely involves electrical components like the pressure switch, inducer motor, or auxiliary limit switch, which require a technician to test safely. Working inside the furnace cabinet involves live voltages and gas components, so professional service is recommended for anything beyond basic airflow checks.

Repair costs vary widely depending on the root cause. A simple filter replacement or vent cleaning may cost very little, while replacing a pressure switch typically runs in the range of $150 to $350 including labor. An inducer motor replacement is generally more involved, often ranging from $400 to $800 or more depending on the model. A service diagnostic call alone commonly costs between $80 and $150 before any parts are factored in.