Furnace clean and tune. Cycled system. Checked heat exchanger. Found some rust and corrosion in the chambers. No cracks found. Checked, removed, and cleaned burners and flame sensor. Found burners very rusted and corroded. Cleaned as best as I could. Igniter is showing be be getting worn. Igniter is showing whiting in a few areas on igniter. Tested normal at this time. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain line. Preformed combustion test and found normal level in exhaust. No CO found in airstream. System is showing signs of getting worn, but operating normal at this time. Yearly maintenance is recommended to help the longevity of this unit. Job complete.