A method and apparatus are provided for diagnosing faults in a monitored system. Estimates of parameter data are generated from the system with reference parameter data characteristic of known behavior of the system. The generated estimates of the parameter data are compared with measured parameter data. A residual is determined for each of the parameters based on a relationship between the estimates of the parameter data and the measured parameter data. A fuzzy classification rule is implemented to determine a likelihood that a predetermined fault exists by using residuals for parameters used to indicate the existence of the predetermined fault.