This article proposes a current path tracking approach that uses a current path state to correlate device status and operating mode of an inverter under normal and abnormal conditions. Based on graph theory, a node-path modeling method is proposed and applied in the T-type inverter topologies. The model reflects system state transitions, given healthy or faulted transistors. Effective conduction paths and node potentials can be derived with the model to indicate circuit faults. In addition, a hierarchical diagnosis method is proposed for single-transistor open-circuit (OC) faults based on estimated output voltage vectors. The diagnosis involves two steps. First, an OC fault can be detected by following a residual between an actual voltage vector and its reference voltage vector calculated from duty ratios of intended current paths. Next, faulty transistors sharing similar features can be distinguished by adopting a current path state reconfiguration approach. Diagnosis variables can be obtained from the controller, and their errors, because of sampling, signal delay, and dead time, are taken into account to avoid misdiagnosis. Experimental results verify the robustness and effectiveness of the proposed method.