Reengineering, or redesign, of electronic circuits is an increasingly important problem in the field of design automation. It is possible that due to a misunderstanding of the original design specifications or a failure to incorporate changes specified in the design document, errors can be introduced into a design. It is also the case that in many situations, there is incomplete or missing information about the design. The redesign problem we consider assumes that original functionality of a design must be reconstructed when faced with either design errors or missing information This paper introduces a methodology for redesign. Based on a system design model for fault diagnosis and formal methods approaches that have been developed, a methodology for reconstructing functionality is proposed. The basic ideas of the approach are illustrated, describing the underlying system model and the algebraic manipulation that is needed to deduce functionality in the presence of missing information.