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]
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.
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]
