An understanding of how sensemaking unfolds when elementary students engage in engineering design tasks is crucial to advancing engineering teaching and learning at K-12 levels. Sensemaking has been widely studied in the context of science as a discipline. In this paper, we seek to contribute to the more nascent efforts to build theory about the characteristics of sensemaking in elementary school engineering. We report on an interpretive case study of a 3rd grade student team who worked with minimal adult intervention to design a solution to an engineering challenge. Considering the entire trajectory of their design process, from the given problem to the solution, we observed that they navigated through multiple epistemic conflicts while making decisions that informed their final solution. We found that these conflicts served as opportunities for sensemaking and that exploring how the students resolved conflicts shed light on their sensemaking processes. Analysis of the team's navigation through epistemic conflicts to come to a design decision helped identify two distinct kinds of engineering sensemaking: student engagement in functional reasoning as they suggested design ideas, and student engagement in mechanistic reasoning as they interpreted test results. Both processes facilitated knowledge building, which in turn supported students' engineering design decisions.