Furnace clean and tune. Cycled system. Checked heat exchanger, found rust and corrosion in all chambers. No visible cracks found with camera. Checked, removed, and cleaned burners and flame sensor. Both ok. Igniter tested ok. Checked and tightened electrical connections and clamps. Cleared drain line. Checked and adjusted gas pressures to maximize efficiency. Preformed combustion test and found normal levels in exhaust. No CO found in airstream. Average life expectancy for this unit is 14 years. System is operating normal at this time, but future fail is expected. Job complete.