An intermittent fault, often called simply an "intermittent", is a malfunction of a device or system that occurs at intervals, usually irregular, in a device or system that functions normally at other times. Intermittent faults are common to all branches of technology, including computer software. An intermittent fault is caused by several contributing factors, some of which may be effectively random, which occur simultaneously. The more complex the system or mechanism involved, the greater the likelihood of an intermittent fault.
A simple example of an effectively random cause in a physical system is a borderline electrical connection in the wiring or a component of a circuit, where (cause 1, the cause that must be identified and rectified) two conductors may touch subject to (cause 2, which need not be identified) a minor change in temperature, vibration, orientation, voltage, etc. (Sometimes this is described as an "intermittent connection" rather than "fault".) In computer software a program may (cause 1) fail to initialise a variable which is required to be initially zero; if the program is run in circumstances such that memory is almost always clear before it starts, it will malfunction on the rare occasions that (cause 2) the memory where the variable is stored happens to be non-zero beforehand.
Intermittent faults are notoriously difficult to identify and repair ("troubleshoot") because each individual factor does not create the problem alone, so the factors can only be identified while the malfunction is actually occurring. The person capable of identifying and solving the problem is seldom the usual operator. Because the timing of the malfunction is unpredictable, and both device or system downtime and engineers' time incur cost, the fault is often simply tolerated if not too frequent unless it causes unacceptable problems or dangers. For example, some intermittent faults in critical equipment such as medical life support equipment could result in killing a patient or in aeronautics causes a flight to be aborted or in some cases crash.
If an intermittent fault occurs for long enough during troubleshooting, it can be identified and resolved in the usual way.
Some techniques to resolve intermittent faults are: