5 Whys

Ask why the defect was made, why it left, and why the system allowed both. Stop at a cause you can change. The example ends on an unapproved fixture shim, which is a system cause, not a person.

Each path has its own action. A chain with no action is not finished.

Each path is drawn as a causal chain from the problem to the last answer. The last answer is a candidate root cause until you record the evidence that confirms it. A skipped why shows as a gap, never as a link.

Keep one chain for how the defect was made, one for how it left the process, and one for the system that allowed both. Stop when the next why would only rename the same fact. The action belongs on the last cause you can actually change.