In this paper, a fast and efficient open-circuit switch and clamping diode fault diagnosis for Neutral-Point-Clamped inverters is proposed. Multilevel converters employed in safety-critical applications should include fault tolerant capability, where fault diagnosis plays a vital role. The proposed fault diagnosis algorithm is based on failure mode analysis, which is a logic based approach. The required signals and measurements are available in the healthy control system. Furthermore, component modeling, load parameters and complicated calculations are not required. The fault detection and identification of the faulty switch or clamping diode are carried out based on the available data such as switching pattern and terminal voltage. Although switches are more fragile than clamping diodes, clamping diodes are also subjected to breakdown. Hence, it is also crucial to achieve a clamping diode fault diagnosis strategy. Moreover, for fault detection procedure, a voltage quantifier is proposed to avoid any misdiagnosis arising from measurement errors and voltage drop in the circuit. The simulation and experimental results are presented to validate the effectiveness of the proposed approach.