The massive deployment of electronic components in various transportation systems has increased the complexity of their embedded networks. Power line communication is a good candidate for reducing cable bundles in these systems. Even if the number of cables is reduced, these cables tend to encounter faults that can lead to total system failure. Therefore, it is critical to design and implement an effective soft fault detection system. In this paper, a Y-shaped network consisting of one source and two receivers is studied. A soft fault, which can be caused by a localized degradation of the cable quality, is represented by a series resistance. The transmission transfer function measured at each receiver is monitored. A new health indicator inspired by the classical cable chain matrix model is proposed. This indicator is calculated at each receiver. A residual based on the calculated indicator is then used along with a preconstructed topology dependent signature matrix to detect a soft fault and to locate the affected branch. Real data extracted from a test bench are used to validate our approach. The results confirm the ability to use Power Line communication systems for diagnostic purposes in addition to their initial implementation purpose.