1. A method to determine a status of an apparatus, comprising: capturing by one or more sensors a signal generated by the apparatus to be processed by a processor in a learning mode; the processor automatically defining a feature by using the signal, wherein the feature includes a representation of a characteristic of a signal in a transform domain; the processor learning a reference model in a learning mode that defines a normal operational mode of the apparatus in a first feature space; capturing a signal generated by the apparatus by the one or more sensors in an operational mode to extract an operational feature; and comparing the operational feature to the reference model to determine whether the apparatus is operating in the normal operational mode and if a decision criteria is met, then analyzing the signal in a second feature space that is different from the first feature space to determine whether there are additional features that define the normal operational mode.