In many domains successful execution of plans requires careful monitoring and repair. Diagnosis of plan execution supports this process by identifying causes of plan failure.

Most plans have to satisfy temporal constraints. An important and common occurring problem during plan execution are violations of temporal plan constraints. This paper addresses diagnosis of such temporal constraint violations by modeling the temporal aspects of a plan as a Simple Temporal Network (STN). We investigate the computational properties of standard diagnostic concepts but we also argue that traditional notions of preferred diagnoses such as minimum diagnosis are not adequate. A new notion of a maximum confirmation diagnosis is introduced.