The invention refers to an apparatus (1) for testing a circuit (2), comprising: an interrupter (12) configured for interrupting based on a circuit model (11) describing at least a part of the circuit (2) a connection (22) between two components (20, 21) of the circuit (2), wherein the circuit model (11) describes the two components (20, 21) connected by the connection (22), an inserter (13) configured for inserting based on the circuit model (11) a test element model (14) into the interrupted connection (22), and an evaluator (15) configured for evaluating based on the circuit model (11) and the test element model (14) a response of the circuit model (11) to the inserted test element model (14). The invention also refers to a corresponding method.