Furnace clean and tune. Cycled system. Checked heat exchanger. Some surface rust and corrosion found, no visible cracks found. Checked, removed, and cleaned burners and flame sensor. Igniter tested normal. Burners were very corroded. Checked and tightened electrical connections and clamps. Found some loose drain lines. Tightened as best as I could. Cleared drain line. Checked and adjusted gas pressures to maximize efficiency. Preformed combustion test. Normal levels found. No CO found in airstream. System operating normal at this time. Job complete.