Much confusion over cooking sausages.
Fat breaks at about 160-165. As Kyle said, a good sausage is a careful mixture of meat with fat suspended in it. So the goal is to cook the meat and brown the casing while not rendering much of the fat out. In fact, something like a thuringer, you should still see little pieces of backfat suspended in the meat.
Medium-low saute in a covered pan for me. Don't let the skin burst but I will stick a Thermapen in to check temps. Natural casings will quickly seal a little pin hole like that. I bring to internal temp of 155 with the natural casings nicely browned. Nothing like the perfect texture of a sausage you stuffed with the snap of a well cooked natural casing when you bite into it.