Functional failure The inability of an asset to carry out one or more intended functions to a level of performance that is acceptable to the asset's user is known as a functional failure. Functional failure is evidently the absence of any function at all. The inability to execute at the level that has been designated as satisfactory performance is also considered a functional failure. Functional failure might cause the system to corrode and suffer severe harm. Reliability failure Reliability is the likelihood that failure will not occur during the interest period, when more than one failure may occur, for objects that are fixed when they fail.