Disturbances in the form of oscillations are usually originated in process plants due to various faults such as sensor faults, valve faults, process faults and controller tuning faults. Many of these faults can be represented as nonlinearities. Faults in the form of nonlinearities may produce oscillations with a fundamental frequency and its harmonics. This study presents a novel method based on the estimated frequencies, amplitudes and phases of the fundamental oscillation and its harmonics to troubleshoot or isolate the root cause of plantwide or unit-wide disturbances. Once the root cause is known, the oscillations can be eliminated, and the process can be operated more economically and profitably. The successful application of the method has been demonstrated both on simulated and industrial data sets.