Abstract: Invention concerns a machine fault diagnostic system. Sensors are provided for sensing an operational parameter of the machine over time in use and outputting corresponding sensor signals. A machine operation diagnosis tool is arranged to receive the sensor signals and has a data store for operational parameter data indicative of a normal mode of machine operation and operational parameter data indicative of one or more known machine faults. The diagnosis tool has one or more processor arranged to compare the received sensor signals with the operational parameter data in the data store in order to determine a match with either the normal mode of operation or the one or more known machine fault, and wherein the processor sentences an unknown fault in the event that said match is not established. The system may record unknown faults and update the diagnosis tool as unknown faults become recognizable faults.