Abstract: A method of training a diagnostic model for identifying a fault condition in a vehicle system includes receiving a plurality of vehicle fault condition data sets, each being associated with a known fault condition of a vehicle system. The vehicle fault condition data sets each include audio data representing an audio signal generated by a microphone during operation of the vehicle system having the known fault condition; and operating data indicating an operating state of the vehicle system. Each vehicle fault condition data set is processed. A frequency domain representation of the audio signal is generated and analysed to identify at least one fault indicator component corresponding to the known fault condition. The diagnostic model is trained to identify the at least one fault condition in dependence on the identification of the at least one fault indicator component in each vehicle fault condition data set.