Abstract: The invention provides, particularly although not exclusively in the context of a fuel injection system of a compression-ignition internal combustion engine, a method for detecting anomalous behaviour of a dynamic system (40), the method including i) determining a system model including plurality of characteristic parameters to define the dynamic system (40), ii) calculating one or more metrics indicative of the current system performance based on the plurality of characteristic parameters, iii) comparing the one or more derived metrics with one or more predetermined metrics indicative of anomalous system behaviour and iv) identifying a predetermined system fault condition if one or more of the derived metrics corresponds to one or more of the predetermined metrics. The invention also provides an apparatus for implementing the aforesaid method.