This study examines the failure of the sanitary sewer network in a city of 200 thousand residents in the Subcarpathian province from 2010 to 2023. Based on operational data, failures are classified in terms of their causes, pipe material, diameter, and location. The analysis reveals that blockages are the most common failure type, accounting for approximately 90% of incidents each year. The average failure rate considering only technical failures is 0.036 no. of failures·km−1·year−1, remaining low at below 0.1 no. of failures·km−1·year−1. The material composition of the network is dominated by PVC pipes (62%), which also exhibit the lowest failure rate (0.003 no. of failures·km−1·year−1), reinforcing their superior durability. In contrast, concrete and stoneware pipes show higher failure rates and structural deterioration, with nearly 60% of failures involving leaks or fractures in concrete pipelines. PE pipes, with a failure rate of 0.013 failures·km−1·year−1, demonstrate significantly better performance than traditional materials. Also, seasonal and random fluctuations were taken into account. Seasonality indicators show a significant increase in failures during spring and summer, which may be related to increased rainfall and water consumption. In contrast, the analysis of random fluctuations allows for the assessment of unpredictable events affecting the network’s functionality, such as sudden failures caused by mechanical loads or infrastructure damage. The findings support sewer network management strategies by optimizing maintenance and modernization efforts, helping to minimize failure consequences. The data used for this study were provided by the water and sewage company.