This paper illustrates how theories (contexts), fail branches, and the ability to control the construction of proofs in MetaProlog play an important role in the expression of the fault diagnosis problem. These facilities of MetaProlog make it easier to represent digital circuits and the fault diagnosis algorithm on them. MetaProlog theories are used both in the representation of digital circuits and in the implementation of the fault diagnosis algorithm. Fail branches and the ability to control their construction play a key role during the construction of hypothesises to explain the fault in a given faulty circuit.