Furnace clean and tune. Cycled system. Checked heat exchanger, found some rusting and corrosion in all chambers. No visible cracks found with camera. Checked p, removed, and cleaned, burners and flame sensor. Checked igniter, tested ok. Checked and tightened electrical connections and clamps. Cleared drain lines. Checked and adjusted gas pressures to maximize efficiency. Preformed combustion test and found normal levels in exhaust. No CO found in airstream. System operating normal at this time. Job complete.