One of the most powerful aspects of the AIAG-VDA 7-Step FMEA methodology is the structured cascading relationship between Cause → Mode → Effect.
👉 This chain answers three critical questions:
- Why does it happen? → Failure Cause
- How does it fail? → Failure Mode
- What happens if it fails? → Failure Effect
Understanding this relationship ensures that risks are traced from the root cause to the end customer impact, allowing teams to set correct Severity, Occurrence, and Detection ratings.
1. Failure Cause → Why it happens #
The cause is the reason the function does not meet its requirement.
- Always actionable.
- Directly linked to occurrence ratings.
Example – Bolting PFMEA:
- Cause: Torque wrench calibration drift.
2. Failure Mode → How it fails #
The mode is the way in which the function fails.
- Describes the deviation from the requirement.
- Connects cause to effect.
Example – Bolting PFMEA:
- Failure Mode: Under-torque (<95 Nm).
3. Failure Effect → What happens next #
The effect is the consequence of the failure mode on local, next-level, or end-user performance.
- Directly linked to severity rating.
Example – Bolting PFMEA:
- Effect: Suspension loosens during driving → vehicle safety risk.
The Cascading Chain – Visual #
Failure Cause → Failure Mode → Failure Effect
(Why) (How) (What Happens)
📌 Example – Electric Motor DFMEA
- Cause: Insulation breakdown in winding.
- Mode: Open circuit in motor winding.
- Effect: Motor does not rotate → vehicle cannot start.
Why This Cascading Relationship is Critical #
- Traceability: Links technical root cause to customer impact.
- Risk Evaluation: Severity is tied to effect; occurrence is tied to cause.
- Action Prioritization: Helps teams decide whether to focus on design, process, or control improvements.
- Prevention-Oriented: By addressing the cause, the mode and effect are automatically mitigated.
Case Study – PFMEA for Welding Process #
- Function: Join two steel sheets with weld.
- Requirement: Weld strength ≥ 5 kN.
- Cause: Electrode wear reduces heat transfer.
- Mode: Weld nugget not formed.
- Effect: Joint breaks under vibration → vehicle structural failure.
👉 The cascading chain showed how a simple process cause (electrode wear) could lead to a safety-critical customer effect. This drove the team to implement electrode monitoring and preventive maintenance.
Best Practices for Applying Cause–Mode–Effect Relationship #
- Always write causes, modes, and effects at the same level of detail.
- Ensure one-to-one logical flow (cause explains mode, mode leads to effect).
- Review the chain with cross-functional team to confirm accuracy.
- Document effects at all three levels (local, next-level, customer).
- Use the chain to decide where prevention and detection controls are needed.
Common Mistakes to Avoid #
- Mixing causes with modes (e.g., “loose bolt” as both cause and mode).
- Ignoring customer effects and focusing only on local issues.
- Writing vague descriptions that cannot be linked (e.g., “bad part”).
- Not validating causes with data or lessons learned.
Key Takeaways #
- Cause = Why, Mode = How, Effect = What happens.
- This cascading chain is the core of FMEA risk analysis.
- Severity is linked to effects, occurrence to causes, and detection to controls on causes/modes.
- A well-structured chain ensures accurate risk prioritization and effective preventive actions.
Next Lesson #
👉 Continue with Lesson 3.6: Step 5 – Risk Analysis (S, O, D, AP)
