Urbanization and demographic growth have significantly increased water demand, leading to the overexploitation of water resources and the discharge of large volumes of wastewater into the environment. One of Algeria's primary challenges regarding wastewater and rainwater management is the degradation of urban sewerage networks. Numerous factors contribute to this issue, making diagnostic research essential. This study focuses on the sewerage networks of Bechar, one of the most important cities in the southwestern region of Algeria. Its objectives are to identify the various causes of sewer system deterioration and to develop a structural and functional diagnosis. A methodology based on axes six main components was used, including video camera inspections, flow measurements, socio-environmental field surveys, and on-site visual observations. The findings indicated that 54.53% of the malfunctions were structural in nature (network obsolescence, cracks, deformations, etc.) while 45.47% were functional, primarily due to sediment buildup. In some areas, a high proportion of extraneous water (32%) leads to network overload, requiring increased maintenance efforts. Additionally, elevated levels of pollution indicators demand that authorities identify and eliminate the specific sources of contamination.