Having a correct mental model of a technical system facilitates interaction and problem solving. To assess such mental models of system functioning, appropriate methods are needed. We tested whether concept mapping with a focus on means-ends relations leads to valid assessments of participants' mental models of system functioning. Automotive and utility vehicle apprentices constructed concept maps of two simple, everyday systems (bike, traffic) and one complex, domain-specific system (fuel temperature control). However, only one group of participants had previously covered the complex system in class. Aspects of participants' concept maps regarding content (correct functional propositions) and structure (intersection over union) were assessed and related to respective reference maps. Results indicated that group differences in knowledge about the complex system were represented by concept map content, but not structure. We argue that the applied structural reference might need to be adapted to match typical requirements of the domain and task.