In this paper, a novel online semiconductor device monitoring scheme is presented. The condition monitoring (CM) scheme is based on measuring the ON-state voltage drop of semiconductor devices, and tracking the changes in their ON-state resistance. The proposed solution measures the ON-state voltage of semiconductor devices at a controlled and readily measurable temperature. This allows for accurate CM of semiconductors as it decouples temperature related and degradation related changes in the ON-state voltage. The temperature decoupling is achieved using natural switching redundancies available to modular multilevel converter systems. Hence, the proposed CM scheme does not interfere with the output voltages and currents generated by the converter.