The invention provides a controller for determining the presence of faults within a fuel injection system of an engine system. The engine system comprises a first injector and at least one further injector, each injector comprising a needle control valve and a pressure control valve. The controller comprises: inputs for receiving data relating to a first engine system parameter; outputs for outputting a control function for controlling the injector valves of each of the injectors, and; a processor for controlling the control function output from the controller. The processor is arranged to output a control function such that the needle control valve of the first injector is kept closed in order to prevent injection from the first injector and to monitor the first engine system parameter as the needle control valve is kept closed, a change in the first engine system parameter indicating that the first injector is operating substantially within normal operating parameters and no or substantially no change in the first engine system parameter indicating that the first injector is faulty.