Abstract: There is provided a method of detecting a fault in a breathing system. The method comprises the steps of (a) taking a series of measurements of a first parameter of the breathing system; and (b) setting a fault boundary for the first parameter, the fault boundary being dependent on a plurality of the measurements of the first parameter. The method further includes at least one update procedure comprising the steps of (c) taking one or more further measurements of the first parameter; and (d) updating the fault boundary, the updated fault boundary being dependent on an updated set of measurements of the first parameter, the updated set of measurements of the first parameter including at least one of the further measurements of the first parameter.