The situation

A customer’s test bench finds 14 wiring harnesses with a terminal pushed out of its connector. The quick answer arrives in minutes: “The operator did not push it in.” Someone writes “retrain” on the action list. Everybody feels better. The defect comes back in March.

The question stopped one step too early.

What 5 Why is for

5 Why is a way to move from a symptom to a cause by asking “why?” again and again. You start with a clear problem. You ask why it happened. You then ask why about the answer. Usually three to five rounds bring you to something in the process, the tooling, or the way work is managed.

The number five is a guide, not a rule. Stop when you reach a cause you can fix, and can prove.

Terms used here:

  • Root cause: a cause that, if removed, stops the problem from returning.
  • Occurrence: why the defect was made.
  • Escape: why the defect was not caught before the customer got it.
  • Poka-yoke: a design that makes the mistake hard or impossible.

A short history

The method is linked to Sakichi Toyoda, founder of Toyoda Automatic Loom Works, in the 1930s. Taiichi Ohno later used it inside the Toyota Production System and wrote about it in Toyota Production System: Beyond Large-Scale Production. Precise details of who first wrote it down are unclear, so we only claim these two names.

Where it applies, and where it does not

Use 5 Why for:

  • a problem you can go and look at, on the line or at the machine;
  • repeated stoppages and small breakdowns;
  • the root cause step of an 8D or A3;
  • defects where people, machine, and tooling all play a part.

Do not rely on it alone for:

  • problems with many interacting variables, where measured experiments are needed;
  • large, complex system failures with hundreds of links;
  • an obvious cause already fixed in five minutes. A fuse that reached the end of its life does not need five whys.

How to do it

The example is fictional. Vistula Automotive Wire & Harness makes the engine bay loom Harness Loom-88. Beata is quality specialist. Robert leads the assembly line. Kamil is the maintenance technician.

  1. State the problem. Write what, where, and when, from what you saw. Terminal pin 4 is unseated in connector housing C-2 on 14 harnesses found at customer test.
  2. Ask the first why. Answer with a physical condition and show evidence. The terminal was not in the secondary lock position. Evidence: returned plugs.
  3. Split into two branches. One asks why the defect was made. The other asks why it escaped.
  4. Go down each branch.
    • Occurrence: Insertion force was high, so the operator felt a click. Crimp wings were flared 0.4 mm over spec (micrometer). The applicator anvil was badly worn (microscope). Applicator maintenance was counted in calendar months, not in strokes (maintenance log). That last answer is a root cause.
    • Escape: The continuity test passed the pin. The probes touch even when the terminal sits 1.5 mm out (fixture drawing). The fixture has no pin presence check. That is a second root cause.
  5. Read it backwards with “therefore”. Start from the root cause and read up. Maintenance by calendar, therefore the anvil wore, therefore the crimp flared, therefore the force rose, therefore the terminal stayed unseated. If a link sounds forced, the chain is broken.
  6. Match actions to root causes. Maintain applicators by stroke count. Add a pull-back or optical check to the test fixture. Notice that neither action is “retrain”.

Three ways it goes wrong

1. Stopping at “operator error”. “Operator forgot” is where the investigation should begin. Ask why the tool allowed it, why the sequence was unclear, and why the next check did not catch it. “Operator error” is not a root cause.

2. Opinion in place of evidence. The team sits in a meeting room and agrees on plausible answers. One unchecked guess at why number two sends the whole chain into fiction. Write “observed” or “assumed” next to every answer, then go and check the assumed ones.

3. One straight line for a branching problem. Real failures usually have at least two parts: a condition that made the defect, and a check that missed it. A single chain finds one and forgets the other, and the customer finds the second one for you.

Our recommendations

  • Go and see. Ask the first why at the machine, with the part in your hand.
  • Always ask the escape question. Even when the cause is clear.
  • Write answers as facts about things. “The anvil was worn” beats “the operator was careless”.
  • Keep the team small and mixed. Include someone who runs the process every day.
  • Containment is not correction. Sorting parts protects the customer now. It does not remove the cause.
  • Check the fix later. Put a date on checking that the problem stayed away.

Try it

Our 5 Why tool is coming soon. It will run in your browser, with no account, and your data will stay on your device. Meanwhile, see the tool page: /tools/five-why.

From the book

This guide follows Chapter 7, the 5 Why chapter, of the field manual. See /book for the book.