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.

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:

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