Fast and reliable fault diagnosis is essential for induction motor (IM) drive systems with high-reliability requirements. In this article, we propose a robust open-switch fault-diagnosis method based on the common-mode voltages of the inverter in the IM-drive systems. This method makes full use of the characteristics of common-mode voltages. First, the calculated common-mode voltages (CCMVs) are equal to each other under normal operating conditions and the CCMVs represent the other behaviors under fault conditions. Second, average common-mode voltage is a degree of freedom in the modulation of the inverter. Based on the first characteristics, the CCMVs are used to detect and locate the open-switch faults of the inverters. Based on the second one, an active common-mode voltage injection method is proposed to improve the credibility of the diagnosis results and reduce the rate of missed detection. The proposed diagnosis algorithm only uses the measured current information, avoiding the application of extra sensors. And its average detection time is about 300 μs, which is three switching cycles. Simulation and experimental results are presented to demonstrate the feasibility and effectiveness of the proposed fault-diagnosis method.