A method of partitioning turbomachine faults, comprising the following steps in the given order: establishing a model of a turbomachine; using the model to determine a modeled gas path parameter (ŷ) and a modeled subsystem parameter ( ẑ); and identifying a fault in the turbomachine; wherein the fault comprises a sensor failure, and wherein the fault is identified based on a comparison of the modeled gas path parameter to a measurement of an actual gas path parameter (y) of the turbomachine and a comparison of the modeled subsystem parameter to an actual subsystem (z) parameter from the turbomachine; characterised in that the method further comprises determining a gas path condition (Δy) representing a difference between an actual gas path parameter (y) and the modeled gas path parameter (ŷ); determining a subsystem condition (Δz) representing a difference between an actual subsystem parameter (z) and the modeled subsystem parameter ( ẑ), and diagnosing a sensor failure based on the gas path condition and the subsystem condition; adjusting the modeled subsystem parameter to match the actual subsystem parameter, determining a model-driven gas path parameter (y z) based on the modeled subsystem parameter, and diagnosing an actuator failure or a module performance change failure based on a comparison of the actual gas path parameter and the model-driven gas path parameter, wherein an actuator failure is diagnosed when the gas path condition is greater than a first gas path threshold value, the subsystem condition is greater than a subsystem threshold value, and the difference between the actual gas path parameter and the model-driven gas path parameter is less than or equal to a second gas path threshold value; and wherein a module performance change failure is diagnosed when the gas path condition is greater than a first gas path threshold value, the subsystem condition is less than or equal to a subsystem threshold value, and the difference between the actual gas path parameter and the model-driven gas path parameter is greater than a second gas path threshold value. establishing a model of a turbomachine; using the model to determine a modeled gas path parameter (ŷ) and a modeled subsystem parameter ( ẑ); and identifying a fault in the turbomachine; wherein the fault comprises a sensor failure, and wherein the fault is identified based on a comparison of the modeled gas path parameter to a measurement of an actual gas path parameter (y) of the turbomachine and a comparison of the modeled subsystem parameter to an actual subsystem (z) parameter from the turbomachine; characterised in that the method further comprises determining a gas path condition (Δy) representing a difference between an actual gas path parameter (y) and the modeled gas path parameter (ŷ); determining a subsystem condition (Δz) representing a difference between an actual subsystem parameter (z) and the modeled subsystem parameter ( ẑ), and diagnosing a sensor failure based on the gas path condition and the subsystem condition; adjusting the modeled subsystem parameter to match the actual subsystem parameter, determining a model-driven gas path parameter (y z) based on the modeled subsystem parameter, and diagnosing an actuator failure or a module performance change failure based on a comparison of the actual gas path parameter and the model-driven gas path parameter, wherein an actuator failure is diagnosed when the gas path condition is greater than a first gas path threshold value, the subsystem condition is greater than a subsystem threshold value, and the difference between the actual gas path parameter and the model-driven gas path parameter is less than or equal to a second gas path threshold value; and wherein a module performance change failure is diagnosed when the gas path condition is greater than a first gas path threshold value, the subsystem condition is less than or equal to a subsystem threshold value, and the difference between the actual gas path parameter and the model-driven gas path parameter is greater than a second gas path threshold value.