In this paper, an open-switch fault diagnosis method for five-level H-Bridge Neutral Point Piloted (HB-NPP) or T-type converters is proposed. While fault tolerant operation is based on three steps (fault detection, fault localization and system reconfiguration), a fast fault diagnosis, including both fault detection and localization, is mandatory to make a suitable response to an open-circuit fault in one of the switches of the converter. Furthermore, fault diagnosis is necessary in embedded and safety critical applications, to prevent further damage and perform continuity of service. The method presented in this paper is based on the switches control orders and the converter output voltage level observation. In HB-NPP and T-type converters, usually some switches are `on' together. Therefore, the fault localization is complicated. The proposed fault diagnosis method is capable to detect and localize in all cases an open-switch fault. Computer simulations are carried out by using Matlab Simulink and SimPowerSystem toolbox to validate the proposed diagnosis.