An apparatus for diagnosing a circuit arrangement for a short circuit and/or a line interruption. The circuit arrangement has at least two load circuits which are connected in parallel. A current measuring device for providing a current signal, which represents the current in the common supply line, is connected in a common supply line to the at least two parallel-connected load circuits. A voltage measuring device detect a node point voltage of each load circuit in the two switching states of the respective switching element. An evaluation device receives the current signal and the node point voltages, converts the current signal into at least one logic current value and the node point voltages into logic voltage values, and infers the type and location of the fault on the basis of prespecified combinations of the at least one logic current value and the logic voltage values.