No fault found

{{short description|Term used in technical maintenance}}

No fault found (NFF), no trouble found (NTF) or no defect found (NDF) are terms used in the field of maintenance, where a unit is removed from service following a complaint of a perceived fault by operators or an alarm from its BIT (built-in test) equipment. The unit is then checked, but no anomaly is detected by the maintainer. Consequently, the unit is returned to service with no repair performed.{{cite journal|last1=Söderholm|first1=Peter|title=A system view of the No Fault Found (NFF) phenomenon|journal=Reliability Engineering & System Safety|date=January 2007|volume=92|issue=1|pages=1–14|doi=10.1016/j.ress.2005.11.004}}{{cite book|last1=James|first1=I.|last2=Lumbard|first2=D.|last3=Willis|first3=I.|last4=Goble|first4=J.|title=Annual Reliability and Maintainability Symposium, 2003 |chapter=Investigating no fault found in the aerospace industry |date=1 January 2003|pages=441–446|doi=10.1109/RAMS.2003.1182029|isbn=978-0-7803-7717-2|s2cid=109886043 }}{{cite journal|last1=Qi|first1=Haiyu|last2=Ganesan|first2=Sanka|last3=Pecht|first3=Michael|title=No-fault-found and intermittent failures in electronic products|journal=Microelectronics Reliability|date=May 2008|volume=48|issue=5|pages=663–674|doi=10.1016/j.microrel.2008.02.003}}

If there is an underlying fault that has not been detected the unit may be returned for repair several times with no fault identified. Alternative descriptors include:{{cite journal|last1=Khan|first1=Samir|last2=Phillips|first2=Paul|last3=Jennions|first3=Ian|last4=Hockley|first4=Chris|title=No Fault Found events in maintenance engineering Part 1: Current trends, implications and organizational practices|journal=Reliability Engineering & System Safety|date=March 2014|volume=123|page=183|doi=10.1016/j.ress.2013.11.003|hdl=1826/9947|url=https://pure.coventry.ac.uk/ws/files/3999358/J1_R.PDF |hdl-access=free}}

• No fault found (NFF)
• Cannot duplicate (CND)
• Fault not found (FNF)
• No trouble found (NTF)
• No defect found (NDF)
• Hidden failures
• False failures