Furnace clean and tune. Cycled system. Checked heat exchanger. Found all chambers have rust and corrosion. No visible cracks found. Checked adjusted gas pressures to maximize efficiency. Checked, removed, and cleaned burners and flame sensor. Checked and tightened electrical connections and clamps. Cleared drain lines. Preformed combustion test and found normal levels in exhaust. No CO found in airstream. Average life expectancy for this unit is 14 years. This system is well over that time period. This system is operating normal at this time but future failure is expected. Job complete.