Furnace clean and tune. Cycled system. Checked and replaced filter. Checked heat exchanger, ok. Removed and cleaned burners and flame sensor, both ok. Igniter tested ok. Checked and adjusted gas pressures to maximize efficiency. Checked electrical connections and clamps. Checked for levels of CO and XS air in exhaust, normal levels found. No CO found in airstream, system operating normal at this time. Job complete.