Furnace clean and tune. Cycled system. Checked heat exchanger. Ok, checked, removed, and cleaned burners and flame sensor. Burners were rusted and corroded. Cleaned back to normal. Checked and tightened electrical connections and clamps. Found drain line clogged. Cleared. Found drain line is pitched slightly uphill. Recommend condensate pump to help drain line to not clog. Checked gas pressures. Preformed combustion test. Normal levels found in exhaust. No CO found in airstream. System operating normal at this time. Job complete. Replaced filter.