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Electronic equipment are subject to degradation and failure as a result of aging and corrosion. Diagnosing to the culprit component, however, is usually not obvious. Some strategic and novel electromagnetic interference (EMI) measurements can be utilized to detect and diagnose failures or degradations. Measuring and monitoring the EMI characteristics of a Unit Under Test (UUT) indicates that the circuit is experiencing an anomaly. The approach is ubiquitous, but in this paper we will focus our discussion to power supplies. Within a power supply, the Fourier series of a full wave rectifier contains only even harmonics of the input waveform. If one of the diodes is degraded or has failed, the output spectral component at the input signal frequency and the odd harmonics of the input will not be zero. An odd harmonic of the input signal frequency will provide an indication of degradation or failure of one of the diodes in the bridge. We present potential measuring and monitoring equipment that can provide in situ non-intrusive prognostic and diagnostic results in operational circuits. The EMI signatures will help pinpoint the culprit component, and repair decisions can be made before the unit is removed from its environment, resulting in substantial savings in support costs.