The 5 whys are the simplest root cause analysis technique there is: starting from the problem, ask "why did this happen?" over and over, using each answer as the basis for the next question, until you reach a cause that, once eliminated, stops the problem coming back. The number five is a reference, not a rule — some analyses close on the third why, others need six.
The technique was born at Toyota and it works because it forces the team to cut through the layers of symptom. The first why almost always gives back a technical symptom ("the fuse blew"); the third starts to expose the process ("the pump was not being lubricated"); the fifth usually lands on how the system is managed ("there is no preventive maintenance plan for that equipment"). Golden rule: the root cause is almost always a missing or faulty process, never a person.
The classic mistakes: stopping too early (staying with the technical symptom and replacing the part without changing the process); skipping the logic (answering a why with something that does not follow from the previous answer); and ending at "human error" or "lack of attention" — if the analysis ends in blame, it has not ended; ask why the process allowed the human error. A useful test is to read the chain backwards with "therefore": each link has to lead logically to the next one.
The template below records the full analysis with the chain of reasoning, the stated root cause and the action linked to it. Keep this record as an attachment to the NCR or the 8D — auditors value seeing the reasoning, not only the conclusion.