Functional block diagrams and reliability block diagrams are critical to the success of FMEAs. Functional block diagrams illustrate the operation and interrelationships between functional entities of a system as defined in engineering data and schematics, thereby providing functional flow sequences for the systems and each indenture level of analysis.

Reliability block diagrams define the series dependence or independence of all functions of a system or functional groups for each life-cycle event, thereby providing identification of interdependencies that can be used for functional FMEAs. If functional block diagrams and reliability block diagrams are not developed for each item configuration in a system during system definition, they must be generated immediately after the system has been defined.

To present the system as a breakdown of its major functions, several functional and reliability block diagrams are usually required, especially if alternative modes of operation must be displayed. These diagrams must show and clearly label all system inputs and outputs, and each block must be assigned a consistent and logical item number that reflects the functional system breakdown order. These numbers are used during the preparation of the FMEA and provide for tracing failure mode effects through all levels of indenture during the analysis as well as when maintaining its accuracy after the system is produced and in use.