Furnace clean and tune. Cycled system. Checked heat exchanger. Found all chambers are very rusted and corroded. No visible cracks found with camera, but several areas that will crack in future. Checked igniter and flame sensor, ok. Checked, removed, and cleaned burners. Ok. Checked and adjusted gas pressures to maximize efficiency efficiency. Preformed combustion test and found high levels of XS air in exhaust. No CO found in airstream. Found ventor motor bearings are worn and noisy. Average life expectancy for this unit is 14 years. This unit is 12 years old and has several parts that are showing signs of getting worn. Due to age, condition, and cost of repair, second opinion recommended for options. Job complete.