A design method is presented which integrates control actionand fault detection and isolation. Control systems operating underpotentially faulty conditions are considered. The problem of designinga single unit which handles both the required control action, as wellas identifying faults occuring in actuators and sensors is discussed.This unit is able to: (1) follow references and reject disturbancesrobustly, (2) control the system such that undetected failures do nothave disastrous effects, (3) reduce the number of false alarms, and (4)identify which faults have occured. The method uses a type of separationprinciple which makes the design process very transparent, and a frequencydomain QTR H-infinity formulation which makes weight selectionmore straightforward. As a consequence of the separation between controland diagnosis, we shall prove that the controller needs not be detuned inorder to improve the diagnosis capabilities, in contrast to common beliefs.