The Fault Diagnosis Expert System for the Australia Telescope analyses ~12000 items of monitor data every minute that report the health and stability of specific components and signal pathways in the array. These data are divided into signatures which are matched against signatures of known failure modes to diagnose problems with the array. Knowledge about many of the failures is acquired by generating them in earlier tests. The system keeps a six-hour history of the detailed behaviour of all monitor data as well as the visibilities. It archives the data in half-hour intervals, characterising the interval with a small set of robust statistical estimators. An interactive graphical user interface allows the simultaneous display of twenty-four histories of either the 6-hour data or one week of the characterised data, together with options for plotting one history against another and for calculating their robust regression.