For problem solving to serve as an effective learning opportunity, it should involve deliberate reflection, e.g., planning and evaluating the solver's progress toward a solution, as well as self-diagnosing former steps while elaborating on conceptual understanding. While expert problem solvers employ deliberate reflection, the novices (many introductory physics students) fail to take full advantage of problem solving as a learning opportunity. In this paper we will focus on self-diagnosis as an instructional strategy to engage students in reflective problem solving. In self-diagnosis tasks students are explicitly required to carry out self diagnosis activities after being given some feedback on the solution. In this and a companion paper, we will present research exploring the following questions: How well do students self-diagnose, if at all, their solutions? What are the learning outcomes of these activities? Can one improve the act of self-diagnosis and the resulting learning outcomes by scaffolding the activity?