This article proposes an open-circuit (OC) fault diagnosis method for voltage source inverters (VSIs) based on microfeature extraction and a third-order tracking differentiator. The proposed method is designed for complex operating conditions such as variable frequency and variable load. It enables effective diagnosis of both types I and II faults. First, a PWM-aligned masked averaging filter (PMAF) is developed. By constructing two averaging windows within each carrier period and masking the intermediate interval, the proposed filter effectively suppresses noise while preserving fault features. Second, a third-order linear tracking differentiator (LTD3) is employed to extract the second-order derivative of currents. The fault features are mapped into a unified second-order derivative domain. As a result, a unified fault criterion is established, eliminating the need for type-specific detection logic. On this basis, a unified fault diagnosis function is constructed to achieve fault detection and fault localization. Compared with existing methods, the proposed method shows improved diagnostic response speed, and maintains robustness under complex operating conditions, with an algorithmic complexity of $\bm{O}\mathbf{(1)}$ . Both simulation and experimental results validate the effectiveness of the proposed method.