This paper reviews the literature dealing with the nature and characteristics of no-fault found events within maintenance lines. Increasing systems complexities have seen a rise in the number of unknown faults that are being reported during operational service. Units tagged as 'No-Fault Found' are evidence that a serviceable component was removed, and attempts to troubleshoot the root cause have been unsuccessful. This scenario worsens when faults occurring at the component level are intermittent in nature. Here, the paper describes the prominent issues that have persisted across a variety of industrial applications and processes for decades. Some recent developments including standards, financial implications and safety concerns are highlighted.

Purpose -The purpose of this paper is to provide a platform for discussion on the problem of no fault found (NFF) events which continues to plague maintenance operations of complex engineering systems. Design/methodology/approach -The research has been collated from many sources: academic literature, industrial discussions and the authors' experiences. The study defines the NFF problem, its causes, impacts and costs as well as an evaluation of the available scientific research. Findings -The paper identifies a continuing serious problem with NFF; it is not just a technical problem but also encompasses organisations, culture and behaviours. Focusing only on one of these at a time is no longer enough to solve the NFF problem in modern maintenance operations and solutions will require each category to be addressed as an integrated problem. Originality/value -The overall value is a detailed picture of the NFF field seen from both an industrial and academic viewpoint. The originality of the paper is that it articulates and organizes the existing knowledge concerning the NFF phenomena, the value of which is to identify gaps in existing research and knowledge.

This is the second half of a two paper series cover aspects of the NFF phenomenon, which is highly challenging and is becoming even more important due to increasing complexity and criticality of technical systems. Part 1 introduced the fundamental concept of unknown failures from an organizational, behavioral and cultural stand point. It also reported an industrial outlook to the problem, recent procedural standards, whilst discussing the financial implications and safety concerns. In this issue, the authors examine the technical aspects, reviewing the common causes of NFF failures in electronic, software and mechanical systems. This is followed by a survey on technological techniques actively being used to reduce the consequence of such instances. After discussing improvements in testability, the article identifies gaps in literature and points out the core areas that should be focused in the future. Special attention is paid to the recent trends on knowledge sharing and troubleshooting tools; with potential research on technical diagnosis being enumerated.