Diagnosing is an important aspect of teachers' professional competence. To successfully diagnose classroom situations, one has to apply pedagogical content knowledge (PCK) in diagnostic activities (DA). Simulated classroom situations with embedded scaffolding can be an effective way of practising skills needed to foster diagnostic competences. In an intervention study with pre-post-test design using the video-based learning environment DiKoBi, we investigated the effects of scaffolds on the diagnostic competences of 57 pre-service biology-teachers in an early stage of university education. For the intervention, we developed two types of scaffolds: one providing additional PCK (PCK-scaffold) and one focusing on the diagnostic activities (DA-scaffold). Pre-service teachers were divided into four groups depending on the type of scaffold they received during the intervention (PCK-, PCK + DA-, DA- and control group). Pre-service teachers receiving PCK-scaffolds showed statistically significantly higher diagnostic competences in the post-test than in the pre-test. This includes the PCK- as well as the PCK + DA-group. For the other two groups, no significant improvement can be reported. Thus, we assume that the diagnostic competences of pre-service biology teachers can effectively be fostered by including PCK-scaffolds in "DiKoBi" and hence a minimum of PCK is necessary for successfully diagnosing in simulated classroom situations.