Abstract: The invention relates to a method for supervising the functioning of a plurality of valve units, a system for performing the method, a control unit and a control assembly for cooperation with the control unit. Each valve unit in the system comprises a semiconductor element (THY1-THYN) and a control unit for controlling the semiconductor element. The control unit comprises a trigger order input for receiving a trigger order (FP), an indication output for emitting a first indication signal (TP) and a trigger signal output (100) for emitting a trigger signal to the semiconductor element. The method comprises the steps: detecting a positive forward voltage (U.sub.T) across the semiconductor valve, generating a first indication signal (IP) when the forward voltage (U.sub.T) exceeds a first threshold value (U.sub.1), generating a trigger signal upon detected receival of the triggering order signal (FP), and the step: generating a second indication signal (SSTP) when the forward voltage (U.sub.