5 Why and fishbone builder for 8D D4

List potential causes on a 6M fishbone, mark each one verified or ruled out with evidence, then run two 5 Why chains: one for why the defect was made (occurrence) and one for why it was not detected (escape). The builder writes a D4 section you can paste into your 8D report. A filled fictional example is loaded; clear it to start.

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Problem statement (from D2: what, where, when, how many)

Fishbone: potential causes (6M)

List every candidate, then mark it verified or ruled out with evidence. Only verified causes belong in the D4 conclusion.

5 Why: occurrence (why was the defect made?)

Why 1Why 2 (why did the line above happen?)Why 3 (why did the line above happen?)Why 4 (why did the line above happen?)Why 5 (why did the line above happen?)Why 6 (why did the line above happen?)

The last filled why is taken as the root cause.

5 Why: escape (why was it not detected?)

Why 1Why 2 (why did the line above happen?)Why 3 (why did the line above happen?)Why 4 (why did the line above happen?)Why 5 (why did the line above happen?)

The last filled why is taken as the root cause.

Systemic cause (optional: why the PFMEA, control plan or review did not prevent it)

Checks (0 open)

No issues found by the keyword checks. That does not prove the cause is right.

D4 text for your 8D report

D4 - Root cause analysis

Problem: Customer found 14 of 2,000 brackets (lot 2610-3) with loose bushing (push-out force below 1.5 kN), line 2, week 41.

Potential causes considered (fishbone / Ishikawa, 6M):
- [Man] Setter did not check limit after tool change - suspected
- [Machine] Press-fit force monitor limit set too wide after tool change - verified; evidence: Reproduced: 20/20 parts at 1.2 kN accepted by monitor; limit record shows 0.8-3.0 kN instead of 1.8-3.0 kN
- [Material] Bushing OD at lower tolerance - ruled out; evidence: 30 retained parts measured, all within drawing

Occurrence - why was the defect made?
  Why 1: Bushing was pressed with too little force
  Why 2: Press-fit force was below spec and the monitor did not stop the press
  Why 3: Monitor lower limit was 0.8 kN instead of 1.8 kN
  Why 4: Limit was re-entered by hand after the tool change on 6 Oct
  Why 5: Tool-change instruction has no step to verify monitor limits against the control plan
Occurrence root cause: Tool-change instruction has no step to verify monitor limits against the control plan

Escape - why was it not detected?
  Why 1: Final push-out test did not detect the loose bushings
  Why 2: Push-out test is sampled 1 per 500 parts
  Why 3: Control plan relies on the force monitor as 100 % detection
Escape root cause: Control plan relies on the force monitor as 100 % detection

Systemic cause: PFMEA rated detection 2 for the force monitor without a limit-verification control after tool change.

Verification: Press-fit force monitor limit set too wide after tool change (Reproduced: 20/20 parts at 1.2 kN accepted by monitor; limit record shows 0.8-3.0 kN instead of 1.8-3.0 kN).
Ruled out: Bushing OD at lower tolerance.

No open points flagged by the builder's keyword checks.
Generated with the 8D Gate root cause builder (https://8d-gate.vercel.app). Advisory text from simple keyword rules; it does not verify your causes and is not a customer or audit decision.

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What the checks look for

Both an occurrence and an escape root cause are present.
The root cause does not blame a person ("operator error", "inattention", "forgot") and is not an action such as retraining.
The root cause is not worded as a guess ("probably", "likely", "assumed").
Chains are not cut short (fewer than 3 occurrence whys or 1 escape why is flagged).
At least one fishbone cause is verified, and every verified cause has evidence.

What it does not do

The checks are simple keyword and completeness rules in English. They cannot tell whether your cause is technically right, whether the evidence is enough, or whether the chain is logical. Five whys is a guide, not a rule: stop when you reach a cause you can verify and control. Your customer's 8D format and requirements decide what D4 must contain. The output is advisory text, not a customer or audit decision.

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