Furnace clean and tune. Cycled system. Checked heat exchanger. Found minimal rusting and corrosion. Removed, checked, and cleaned burners and flame sensor. Both are ok at this time. Checked and adjusted gas pressures to maximize efficiency. Checked electrical connections and clamps. Both are tight. Normal levels of CO and XS air in exhaust. No CO found in airstream. System operating normal at this time. Job complete.