Lennox Furnace Error Code 2+4

The 2+4 error code means the furnace control board detected the low fire pressure switch in a closed state before the combustion air inducer motor was turned on. This is not normal, since the pressure switch should only close after the inducer creates sufficient draft vacuum. The furnace locks out as a safety precaution when this condition is detected. The most common cause is a pressure switch that has stuck in the closed position.

The 2+4 code is a pre-ignition fault, meaning the furnace shut down before any combustion attempt. The control board caught the abnormal pressure switch signal during its startup self-check and prevented the system from proceeding, which is the safety system working as intended. While the immediate risk is limited, a stuck or failed pressure switch should still be repaired promptly because a malfunctioning pressure switch cannot reliably perform its safety monitoring role during normal operation.

Repair costs depend on the root cause. If only the pressure switch needs replacement, parts and labor typically fall in the range of $100 to $300 depending on your location and the specific switch required. If the issue involves wiring repair or control board replacement, costs can range from $300 to $600 or higher. Getting a diagnostic visit from a licensed HVAC technician will give you an accurate estimate based on your specific situation.

Some basic checks are within reach for a knowledgeable DIYer, such as visually inspecting the pressure switch hose for kinks, blockages, or moisture and clearing any condensate from the tubing. However, confirming whether the pressure switch itself is defective requires a multimeter and familiarity with furnace wiring, and replacing gas furnace components involves safety risks. If you are not experienced with HVAC systems, it is best to have a qualified technician handle the diagnosis and repair.

Resetting the furnace by cycling the power may temporarily clear the fault and allow the furnace to attempt another startup, but if the underlying problem persists, the 2+4 code will return. A stuck closed pressure switch, for example, will trigger the fault on every startup attempt until the switch is replaced. Resetting is useful for confirming whether the issue is intermittent, but it is not a fix for a defective component.