Furnace clean and tune . Cycled system. Checked heat exchanger. Found deep etching forming in all chambers. These will become cracks at any time. Heavy rusting and corrosion found in all chambers. Checked, removed, and cleaned burners and flame sensor. Found both to be worn down, but in ok condition at this time. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain lines. Found obstruction in drain trap. Cleared this debris out. 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 over that time period and has issues. Future failure is expected. Job complete.