Reasoning about action and change is integral to the diagnosis, testing and repair of many artifacts, and yet there is no formal account of diagnostic problem solving which incorporates a theory of action and change. In this abridged report, we provide a situation calculus framework for diagnostic problem solving. Using this framework, we present results towards a characterization of diagnosis for behaviorally static systems whose state can be affected by events which occur in the world, and which require world-altering actions to achieve tests and repairs. Diagnosis is defined more broadly in terms of what happened in addition to the traditional conjccture of what is wrong. Observations of events and actions in the world are used to diagnose some system malfunctions. By developing our characterization in terms of the situation calculus, we are able to contribute towards a formal characterization and semantics for this broader notion of diagnosis, testing and repair, in addition to dealing formally with issues such as the frame problem.