A method of monitoring a system is disclosed, in particular to identify the cause of conditions outside expected operating conditions. The output of one or more sensors associated with a system is monitored and data from the one or more sensors is arranged as a plurality of modes with each mode being defined by a different condition in which the system may operate. Faulty conditions are identified by monitored data being outside one of the plurality of modes. The use of a plurality of modes enables operation of the system to be defined and tracked more precisely and false alarms may be reduced. At least one of the modes may be established to indicate a particular failure of the system. This failure mode may have a likely cause of the failure associated with it such that diagnosis and repair may be facilitated quickly and easily.