We describe a manufacturing screening task, the diagnosis of defects on a computer board. Then we describe several strategies used by expert troubleshooters performing the task. These strategies use "inexact models" of the components and connections on the board. A prototype expert system has been implemented that uses the strategies and models. The strategies and models are robust because they are applicable to a wide range of problems, including problems not previously encountered. Brittleness is a common problem with diagnostic systems. We propose the use of these types of strategies to increase the robustness of diagnostic systems.