Try it while you read
Open the free 5 Why Analysis tool and click Load sample: it reproduces the case study below.
Theory: depth, not breadth
The 5 Why technique, from the Toyota production system, asks “why?” repeatedly until the answer is a cause that, when removed, prevents the problem. Its strength is depth: it goes past the symptom (“the diameter was too large”) to the system cause (“the instruction did not include the offset entry”).
In automotive problem solving two chains are expected: occurrence (why was the defect made?) and escape (why did the controls not detect it?).
Key point
Step by step
- Write the problem with facts: what, where, when, how many.
- Answer each why with something you can check.
- Stop at the first actionable system cause.
- Repeat for detection.
- Prove each root cause and record the evidence; send everything to D4 of the 8D report.
Case study: oversize bearing seats
The Load sample analyses 38 of 500 shafts found oversize by the customer.
- Occurrence: diameter turned too large → offset wrong after insert change → offset not entered → not in the instruction → instruction written for the old machine. Root cause: the tool change instruction lacks the offset entry.
- Escape: not detected → only the first part checked → first part measured before warm-up → no check defined after warm-up. Root cause: control plan has no check after insert change and warm-up.
- Verification: a trial without offset entry reproduced +0.010 to +0.013 mm; with automatic probing 4,000 parts were made without a defect.
Common mistakes
- Stopping at “human error”.
- Answers based on assumptions rather than facts.
- Only one chain, so the detection gap stays open.
Frequently asked questions
What if there are several causes?
Branch the analysis: follow each answer that is verified as contributing. A fishbone diagram helps to list the candidates first.