Furnace clean and tune. Cycled system. Checked heat exchanger. Found some rust and corrosion. No cracks found. Checked, removed, and cleaned burners and flame sensor. Both ok. Checked and tightened electrical connections and clamps. Cleared drain line. 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.