Prompt detection of power switch failure is crucial for fault-tolerant operation of multilevel converters. The paper presents a new technique of detecting, identifying, and locating a device fault in a multilevel DC-DC flying capacitor (FC) converter. This is realized by continuously analyzing the magnetic component (inductor or transformer) voltage harmonics for failure detection and subsequently identifying failure type and location with look up table method. The harmonics analysis is performed by using discrete Fourier transform. This new approach uses only existing an auxiliary winding in the magnetic core without adding any new hardware. The experimental results validate the proposed concept.