Early and accurate fault detection in modern industrial machines is crucial in order to minimize downtime, increase the safety of plant operations, and reduce manufacturing costs. The process monitoring techniques that have been most effective in practice are based on the analysis of historical process data. In this paper we present a novel approach that uses Kernel Spectral Clustering (KSC) on the sensor data to distinguish between normal operating condition and abnormal situations. In other words, the main contribution is to show how KSC can be a valid tool also for outlier detection, a field where other techniques are more popular. KSC is a state-of-the-art unsupervised learning technique with out-of-sample ability and a systematic model selection scheme. Thanks to the abovementioned characteristics and the capability of discovering complex clustering boundaries, KSC is able to detect in advance the need of maintenance actions in the analyzed machine. © 2013 IEEE.