There are many threshold concepts for students to grasp in Engineering Mechanics. Research has shown that one of the critical concepts that must be understood is the Free Body Diagram (FBD). This paper describes an approach to check students’ mastery of simple Free Body Diagrams and to address errors and misconceptions they may have. An online FBD quiz has been designed and implemented in a first year Engineering subject. Teaching material and follow-up support is targeted for students who struggle with the concept until mastery is achieved. These have been based on the theory of threshold concepts and the force concept inventory. The FBD quiz is found to be a good predictor of future performance in subsequent Engineering Mechanics subjects. An analysis of student performance in the FBD quiz has identified the questions that best test the conceptual understanding.