Abstract: An engine health diagnostic apparatus is provided for analysing health of a reciprocating internal combustion engine. The apparatus comprises feature generation circuitry for processing vibration sensor data received from a vibration sensor detecting vibration at a component of the reciprocating internal combustion engine 4 and generating a feature vector indicating multiple features of the sensor data. Processing circuitry processes the feature vector using a trained classification model which is defined by model parameters characterising a decision boundary of healthy operation learnt from a training set of feature vectors captured during healthy operation of the engine. The model generates an engine health indication providing a quantitative indication of deviation of the feature vector from the decision boundary of healthy operation.