Lennox Furnace Error Code DIAG#1: Flash / DIAG#2: Off

This code indicates a Pressure Switch Watchguard fault, meaning the pressure switch opened while the furnace was actively running. The pressure switch monitors the draft created by the combustion air blower, and if it loses that signal mid-cycle, the control board shuts the furnace down as a safety precaution. The most common triggers are a blocked condensate drain, a restricted vent pipe, or a faulty pressure switch. The furnace will continue to lockout until the root cause is corrected.

No. The Pressure Switch Watchguard is a safety lockout designed to prevent the furnace from operating when combustion airflow is compromised. Running the furnace with a blocked vent or condensate problem could allow combustion gases to accumulate in unsafe locations. Leave the furnace off until the fault is diagnosed and corrected.

If the fix is simply clearing a blocked condensate drain or removing a vent obstruction, out-of-pocket costs can be minimal, typically covering only a service call fee in the range of $75 to $150. If the pressure switch itself needs replacement, parts and labor generally run between $150 and $350 depending on the specific switch and local labor rates. Inducer motor replacement is the most expensive scenario, often ranging from $400 to $800 or more including parts and labor.

Start by checking the condensate drain line and trap for blockages and clearing any standing water or buildup. Next, inspect the inlet and exhaust vent pipes at their exterior terminations for ice, debris, or physical damage. If those are clear, examine the pressure switch hose for cracks or loose connections and test the pressure switch itself for proper continuity and pull-in pressure. If the switch is faulty or the inducer motor is weak, those components will need to be replaced.

Resetting the furnace may temporarily clear the lockout and allow it to attempt another heating cycle, but the code will return if the underlying problem has not been fixed. The Watchguard circuit is designed to detect repeated pressure switch dropouts, and continued resets without resolving the cause can lead to longer lockout periods. Address the root cause before resetting.