Lennox Furnace Error Code 5+2

The 5+2 flash code on a Lennox furnace means the control board (100869) has failed its internal self-check routine. This is an internal error originating within the board's own circuitry and logic, not in an external sensor or component. The board is designed to restart automatically if the fault condition clears on its own. If the error persists and the furnace does not recover, the control board likely needs to be replaced.

When the 5+2 code is active, the furnace control board has halted normal operation because it detected a fault in its own internal state. The furnace will not operate normally in this condition, so the risk of running it is low since it is essentially shut down. However, you should not attempt to bypass the error or force the system to run, as the board's self-check exists to prevent unsafe operation. Contact a technician to diagnose and resolve the fault before relying on the furnace for heating.

If the fix requires replacing the control board (100869), costs typically range from a few hundred dollars to over five hundred dollars depending on part availability, labor rates in your area, and whether additional components are involved. If the cause turns out to be a wiring or power supply issue, costs may be significantly lower since no major parts are needed. Getting a diagnostic assessment from a licensed HVAC technician will give you an accurate estimate for your specific situation.

Some basic checks are within reach for a knowledgeable homeowner, such as verifying the furnace has stable power, checking for tripped breakers, and inspecting visible wiring connections for looseness or corrosion. However, diagnosing a failed control board and replacing it involves working with line voltage and low-voltage control circuits, which is best handled by a qualified HVAC technician. Replacing the control board itself requires sourcing the correct part and properly transferring all wiring connections.

The Lennox control board is designed to restart automatically if the fault condition that caused the 5+2 code resolves on its own, so a power cycle reset may temporarily clear the error if the cause was a transient power event. However, if the control board has a hardware failure, resetting the furnace will not resolve the underlying problem and the error will return. Repeated or persistent 5+2 codes after resets strongly indicate the control board needs replacement.