Lennox Furnace Error Code LED#1 Off / LED#2 Slow Flash

This code indicates a pressure switch fault. The furnace control board has detected that the pressure switch was either open when it should have closed after the inducer blower started, closed before the inducer started, or opened repeatedly during a single heating call (five times or more). The fault points to problems with venting, the condensate drain, the pressure switch hose, or the switch itself.

No, you should not attempt to bypass or ignore this fault. The pressure switch is a safety control that confirms proper venting of combustion gases. If the venting system is blocked, running the furnace could result in carbon monoxide buildup in your home. Leave the furnace off until the root cause has been identified and corrected.

Repair costs vary depending on the cause. If the issue is a blocked vent that you can clear yourself, the cost may be minimal. A replacement pressure switch typically runs in the range of $15 to $75 for the part, plus labor costs that generally range from $75 to $200 depending on your region and service provider. An inducer motor replacement is more expensive, with parts and labor commonly ranging from $300 to $600 or more.

Start by inspecting the inlet and exhaust vent pipes at the exterior of your home for any blockages such as ice, debris, or nests. Next, check the condensate drain line and trap for clogs. Examine the small rubber hose connecting the pressure switch to the collector box for cracks or water. If those components are clear, the pressure switch or inducer motor may need to be tested or replaced by a qualified technician.

A reset may temporarily clear the code and allow the furnace to attempt operation again, but if the underlying problem is not resolved, the fault will return. In many cases the control board will lock out again quickly after restarting because the pressure switch condition is still present. Resetting should only be used as part of a diagnostic process, not as a fix.