A possible application of disturbance analysis to the diagnosis of control system failure is presented. A method is developed which permits detailed diagnosis of the disturbance arising from the failure of one or more pressurizer pressure sensor channels in a PWR, starting from normal operating conditions. For the case analyzed in detail, it is found that combinations of failure ranges in all pressure sensor channels can be clustered into groups according to the induced system behavior. The prediction of system behavior corresponding to each failure group is obtained partly by means of qualitative deductive analysis and partly by means of a reactor thermal-hydraulic simulation code. From the information gathered in this manner, the system symptoms which are needed to uniquely diagnose the initiating failures can thus be derived. The logic, as well as the temporal structure containing the distinguishing sets of system symptoms, can be implemented into a computer code to execute the real time analysis. Although the control system analyzed herein was specific, the methodology may have more general applications.