Furnace clean and tune. Cycled system. Checked heat exchanger. Found rusting 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 adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain lines. Preformed combustion test and found normal levels in exhaust. No CO found in airstream. System is operating normal at this time. Job complete.