I noticed a few weeks back that my chimney seems to be emitting black smoke any time the cat probe was reading below 700F. Above 700F, it would seem all I got was white steam. I'm using the new Condar SteelCat for one year now, which the tech at Condar told me was still a little experimental for them. I hesitated to call Condar, since I wasn't sure if this was my imagination, or if maybe this is normal. A few weeks have gone by since that observation, and this morning I noticed black smoke out the chimney with the cat probe at 814F. I ran it up to 1000F, and now no smoke. Failing cat? It's never been above 1800F, to my knowledge.