This paper describes how the effects of possible component failures on an electrical system can be predicted through simulation of the correct behavior of the system, and repeated simulation of versions of the system containing faulty components. The results of such simulation are useful both for failure mode and effects analysis (FMEA), and for diagnosis. Such results have been used commercially for FMEA for a much longer time than for producing diagnostics. This has led to techniques for reducing the amount of engineer effort needed to repeat the FMEA when changes to the design occur, a modified process we have termed "incremental FMEA". This modified process significantly reduces the amount of engineer effort needed to evaluate the consequences of minor changes to a system design. This paper shows how the results of FMEA are also being used to generate diagnostics, and applies incremental FMEA techniques to the diagnostic challenges of variants and of producing diagnostics as a system evolves over time. The idea of incremental diagnostics should reduce the amount of effort needed to produce different versions of related diagnostics in a similar way to how it reduced the effort needed for repeated FMEAs.