Performed the level 2 furnace cleaning. Burners and flame sensor are clean. Heat exchanger has minor rust. Combustion readings were normal. No CO detected in the air. Drain is clear. High limit passed the safety check. Heat rise was 52°. Unit is heating properly. I was smelling gas by the furnace. I did a courtesy leak search and found the union fitting by the furnace leaking. I shut off the gas to the furnace and took the union apart. I tightened everything down and put the piece back together. I leak checked the fittings again and had no more indication of a leak and the smell was gone.