I am not shure if it will burn correctly without it.On mine there is handle on the right that you pull out and it opens the bypass around the cat for when your getting the fire going and the cat up to temp. When its burnig right the bypass gets closed and the flue gas goes through the cat and gets reburned releasing more heat.I think you can get operating instructions at Englanders web site or if your interested I will let you know how I use mine.You can get a new cat from Condar stove parts and a cat thermomater too. Here are some pics of mine and the cat thermometer on the side.