Furnace clean and tune. Cycled system. Checked heat exchanger, ok. Checked, removed, and cleaned burners and flame sensor. Burners and flame sensor were corroded. Cleaned as best I could. Checked and adjusted gas pressures to maximize efficiency. Checked electrical connections and clamps. Tightened all. Preformed combustion test. All levels were normal in system exhaust. No CO found in airstream. Tested unit safeties. All preformed correctly. System operating normal at this time. Job complete.