A method of determining the nature of a fault in the operation of a hydraulic fuel injector includes sending an actuation pulse to an actuator of the injector, the actuation pulse being above the minimum drive pulse of the injector. The method also includes analyzing a signal from an accelerometer of the injector to determine if there is a high frequency vibration component subsequent to the end of the activation pulse. The method also includes determining the functionality of a needle control valve of the injector dependent on the outcome of analyzing the signal from the accelerometer.