This paper presents a novel diagnosis method based on error voltage for 1GBTs open-circuit faults in dual-loop controlled voltage-source inverters. The phase voltage changes when open-circuit faults occur, and then there are error voltages of each phase which can be calculated directly by control algorithm with phase voltages and their corresponding flexible references both available from the main control system. Therefore, the proposed diagnosis method requires no additional sensors or electrical circuits to detect open-circuit faults, further with the features of fast diagnostic time, being suitable for various load conditions, strong robustness against false alarms and being easily inserted to the existing control algorithms as a subroutine. The experimental results are carried out to show the effectiveness of the proposed method.