No fault found

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.^[1][2][3]

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:^[4]

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

The NFF problem

Depiction of the no fault found cycle. Each clockwise cycle after the initial is a waste of maintenance resource.

As the figure shows once a fault has been reported, investigated, and no fault found any future problems caused by the fault cause additional work which is a waste of maintainer time. Different causes have been suggested for this issue.

• Some can be attributed to the way a possible fault is perceived by the user.
• Some can be attributed to the diagnostic methods available to the maintainer.

The fact remains that no fault found causes a cost to industry. NFF is thought to cost the United States Department of Defense in excess of US$2 billion per year.^[5]

See also

• Troubleshooting
• Unintended consequences