D4 is where many supplier 8D reports come back. The usual problem is not a wrong cause but an unproven one: a label like “human error” or “material issue” with no mechanism and no data. This page shows what weak D4 statements look like and how the same D4 reads once it is verified. It applies to automotive, aerospace, medical devices, electronics, food, and general manufacturing. Your customer’s format comes first. For the method itself, see how to write D4 root cause.
Signs of a weak D4
- It names a person, not a process: “operator error”, “lack of attention”.
- It names a category, not a mechanism: “material issue”, “machine problem”.
- It repeats the symptom in other words: “part out of tolerance because dimension was wrong”.
- It gives only one cause, with nothing on why the defect escaped.
- It has no evidence: no data, no trial, no comparison of good and bad parts.
- The corrective action is retraining or extra inspection, which suggests the real cause was never found.
For the most common case, see why operator error is not a root cause.
Weak vs verified D4: examples from four sectors
The examples below are illustrative and fictional, not from real cases. Each shows a weak D4, a rewritten cause, and the evidence that verifies it.
Automotive, injection moulding (illustrative)
Weak D4: “Root cause: operator did not check the part. Action: operator retrained.”
Rewritten cause: Short shots came from a blocked cavity vent. Venting was not on the preventive maintenance plan, so it was cleaned only when someone noticed.
How it was verified: The cause was reproduced: blocking the vent on a trial gave the same short shot. Shots after the vent cleaning and plan change were inside the limit.
Electronics, SMT assembly (illustrative)
Weak D4: “Root cause: solder paste issue.”
Rewritten cause: Insufficient solder on one connector came from a worn stencil aperture. Stencils had no wear limit or print count in the system.
How it was verified: The worn aperture was measured against a new stencil, and solder paste inspection showed the low volume only at that position.
Medical devices, assembly (illustrative)
Weak D4: “Root cause: human error during labelling.”
Rewritten cause: Two label rolls for similar variants were stored side by side with the same layout, and the line clearance checklist did not ask to verify the roll.
How it was verified: A walk-through showed the wrong roll could be loaded without any check. The new clearance step was then tested on the line.
Food, packing (illustrative)
Weak D4: “Root cause: machine malfunction.”
Rewritten cause: Seal failures came from a heater temperature that drifted after a sensor was replaced, because the new sensor was not calibrated at installation.
How it was verified: Logged temperature data matched the failure times, and seals made after calibration passed the peel test.
Rewrote your D4? Get a free Accept / Conditional / Reject advisory on the report text, with missing evidence named, before it goes to the customer.
8D Gate is a free pre-send readiness advisory. It doesn't replace the customer's decision. We don't store your 8D body in our database; you can delete anytime. Anthropic processes the text for the review.
What turns a cause into a verified cause
- Mechanism: a physical or process explanation of how the cause creates the defect.
- Reproduction: switching the cause on and off brings the defect back and removes it, where that is safe and practical.
- Data: measurements, logs, or good-vs-bad comparisons that match the time and place of the failure.
- Systemic step: why the process allowed the cause, such as a missing maintenance item, limit, or check.
- Escape cause: why your controls did not catch the defect before it shipped. See the escape point in 8D.
A quick rewrite pattern
Write D4 as three short statements: “The defect occurred because [mechanism]. The process allowed this because [system gap]. It was not detected because [escape cause].” Under each, add the evidence: the trial, the data, or the comparison. If a statement has no evidence yet, mark it as a hypothesis and give the date of the test.
Before you send
- D4 names a mechanism, not a person or a category.
- Occurrence and escape causes are both stated.
- Each cause has evidence attached or a dated test.
- The D5 action removes the cause you proved, not a symptom.
- Someone outside the team can follow the logic without asking.
Next: show the action works in 8D D6 verification of effectiveness, or start from the 8D report template. If a report already came back, see 8D rejected? What to do next.
FAQ
- How many whys are enough?
- Stop when the cause is something you can change in the process or system, and when you can show it with data. The count matters less than the evidence.
- What if we can’t reproduce the failure?
- Say so in D4, list the candidate causes you tested and how you ruled them out, and explain what evidence supports the most likely one. That is usually easier to follow than a cause stated without proof.
- Does 8D Gate decide whether our root cause is right?
- No. It is a pre-send readiness check of the report text: it flags a weak or unsupported D4 and asks questions. It doesn’t know your process and isn’t a customer decision. The review tool is v0.1: it reviews English 8D reports, and the interface and report are in English. Templates are also available in German, Czech and Polish.
Free pre-send readiness check
Before the report goes out, run the text through the free supplier 8D readiness check.
Paste text or upload PDF, DOCX, or Excel. You get an Accept / Conditional / Reject advisory, a score, section ratings, missing evidence, and questions to close gaps, with a PDF download. It is a pre-send readiness check for your team, not a customer decision.
8D Gate is a free pre-send readiness advisory. It doesn't replace the customer's decision. We don't store your 8D body in our database; you can delete anytime. Anthropic processes the text for the review.
Get the free 8D / D4 checklist
Keep the one-page version next to your D4. It covers D1–D8 and goes deeper on occurrence, escape, and verification.
Get the free 8D / D4 checklist