Abstract : With the increasing number of sophisticated electronic devices in automobiles, while enhancing vehicle safety and comfort, higher demands are placed on the electromagnetic environment within the vehicle. Conducted interference generated during the operation of numerous onboard relays is one of the main causes of fluctuations in the in-vehicle power grid. This paper proposes a method to collect high-frequency interference signals during relay operation by inserting a line impedance stabilization network between the relay contacts and the vehicle power supply. Then, continuous wavelet transform is used to analyze the interference signals. Based on the submatrices extracted from the wavelet coefficient matrix to construct a local time-frequency diagram, an energy threshold is set for the wavelet coefficient values ​​to obtain a three-dimensional time-frequency-energy diagram. The setting of the energy threshold and the local time-frequency analysis technique provide a good basis for identifying frequencies that cause severe conducted interference and the protective measures to be taken. Keywords : Electromagnetic compatibility; Conducted interference; Continuous wavelet transform; Local analysis technique; Energy threshold 1. Introduction Current research shows that reducing the impact of electromagnetic interference (EMI) in automotive power and electronic equipment has become an important issue. In recent years, new technologies have focused on improving reliability. With the increasing electrification of vehicles, this not only brings safety and comfort but also places higher demands on the automotive electromagnetic environment. One cause of fluctuations in the vehicle's power network during driving is the operation of numerous onboard relays. Therefore, automotive electromagnetic compatibility (EMC) is increasingly important, especially in troubleshooting electromagnetic interference. This paper first describes the measurement equipment and interference signal processing. Then, it compares and analyzes the interference signals using both traditional Discrete Fourier Transform (DFT) and continuous wavelet transform. Based on this, an energy threshold is set using continuous wavelet transform to derive a local analysis technique for the interference signal. This technique can provide much useful information about the interference signal. In particular, the local information analysis provides a better understanding of the frequency content of the interference signal as it evolves over time, improving the accuracy of identifying the interference source. The full text of the study on the application of local analysis techniques in identifying electromagnetically conducted interference noise from onboard relays is available for download.