Furnace clean and tune. Cycled system. Checked heat exchanger. Found some rusting found in all chambers. No visible cracks found with camera. Checked, removed, and cleaned burners and flame sensor. Burners were corroded and rusted. Cleaned as best as I could. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain lines. Ventor motor bearings are ok at this time. Preformed combustion test. Normal levels of CO found in exhaust. High levels of XS air found. This is caused by blockage in secondarily heat exchanger. No CO found in airstream. Average life expectancy for this unit is 14 years. System is worn, but operating normal at this time. Future failure is expected. Job complete.