Furnace clean and tune. Cycled system. Checked heat exchanger, ok. Checked, removed, and cleaned burners and flame sensor. Igniter tested normal. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain line. Preformed combustion test. Found all levels in exhaust were normal. I CO found in airstream. System operating normal at this time. Job complete.