When dealing with the engines or APU, you have fire loops to detect fires and fault detection systems to keep them honest. You have two fire bottles for the engines, one of those also covers the APU. Inside the airplane the trash receptacles have fire bottles and the cabin has a set of fire extinguishers. With the rest of the airplane, bleed air looks for things hotter than 250°F and electrical sensors look for things hotter than 150°F. What follows is a list of the systems and another list of various things that can go wrong.