In this paper, we suggest an architecture for a software agent whichoperates a physical device and is capable of making observations andof testing and repairing the device's components. We presentsimplified definitions of the notions of symptom, candidatediagnosis, and diagnosis which are based on the theory of actionlanguage ${\cal AL}$.The definitions allow one to give a simple account of the agent'sbehavior in which many of the agent's tasks are reduced to computingstable models of logic programs.