A diagnostics subsystem for performing predictive diagnostics on a machine such as a vacuum pump. The subsystem has one or more parameter sensors providing measurable data, a process model for modelling machine operation and generating at least one estimated operating parameter, a comparator for comparing the sensed operating parameter with the estimated operating parameter; discontinuous signal injection means for injecting a discontinuity into the model to maintain sliding mode operation in the model; and means for analyzing the discontinuous injection signal for trends indicative of a fault in the machine.