In Step 4: Failure Analysis of the AIAG-VDA 7-Step FMEA approach, once failure modes and effects are defined, the next step is to determine the causes of failure.
👉 A Cause of Failure answers the question:
“Why does this failure mode happen?”
Understanding causes is essential, because preventive actions in FMEA are always targeted at eliminating or reducing causes.
What is a Cause of Failure in FMEA? #
- A Cause of Failure is the specific reason why a function fails to meet its requirement.
- It should always be actionable — meaning the team can influence it through design improvements, process changes, or controls.
- Causes are different from failure modes:
- Failure Mode = How it fails
- Failure Cause = Why it fails
Types of Causes of Failure #
1. Design Causes (DFMEA)
Causes related to product design weaknesses.
- Material properties not sufficient.
- Incorrect tolerance selection.
- Inadequate thermal, mechanical, or electrical design.
- Software logic errors.
Example – Electric Motor (DFMEA):
- Failure Mode: Motor rotates below 1500 RPM.
- Cause: Magnet strength below specification.
2. Process Causes (PFMEA)
Causes related to manufacturing or assembly processes.
- Incorrect machine settings.
- Tool wear or misalignment.
- Human error in assembly.
- Poor environmental conditions (temperature, humidity, dust).
Example – Welding Process (PFMEA):
- Failure Mode: Weld strength < 3 kN.
- Cause: Electrode wear leading to poor heat transfer.
Examples of Causes of Failure in FMEA #
| FMEA Type | Failure Mode | Cause of Failure |
|---|---|---|
| DFMEA | Seatbelt buckle fails to lock | Weak spring design |
| DFMEA | ECU malfunctions | Software bug in control algorithm |
| PFMEA | Bolt under-torqued | Torque wrench not calibrated |
| PFMEA | Paint coating too thin | Spray nozzle clogging |
How to Identify Causes of Failure – Step-by-Step #
- Start with the Failure Mode (from Lesson 3.5.1).
- Example: “Bolt under-torqued.”
- Ask: Why can this failure mode occur?
- Tool miscalibrated? Operator skipped step? Material defect?
- Drill down to root causes.
- Use 5 Whys or Fishbone (Ishikawa) diagram.
- Ensure causes are actionable.
- Avoid vague answers like “human error.” Instead, define “operator did not follow torque sequence due to unclear work instruction.”
Case Study – PFMEA for Bolting Operation #
- Function: Secure suspension bolt at 100 ± 5 Nm.
- Failure Mode: Under-torque.
- Effect: Suspension loosens, vehicle safety compromised.
- Cause of Failure:
- Torque wrench calibration drift.
- Operator skips torque sequence due to unclear instructions.
- Bolt lubrication inconsistent.
👉 Because causes were identified clearly, preventive actions were targeted: improved calibration schedule, revised work instructions, controlled lubrication.
Why Identifying Causes is Important #
- Drives Preventive Actions: Only causes can be directly controlled or eliminated.
- Improves Risk Prioritization: Occurrence ratings in FMEA are tied to failure causes.
- Supports Continuous Improvement: Causes link FMEA with root cause analysis tools (5 Whys, Fishbone, Fault Tree).
- Customer Safety & Quality: Preventing causes ensures failures do not reach customers.
Common Mistakes in Identifying Causes #
- Writing generic causes (e.g., “bad design,” “human error”).
- Confusing effects with causes.
- Not using data or historical evidence to validate causes.
- Listing causes that are outside team control (e.g., “poor supplier material” without supplier input).
Best Practices for Identifying Causes of Failure #
- Always link causes directly to the function and failure mode.
- Use cross-functional teams for brainstorming causes.
- Apply structured tools like 5 Whys, Fishbone, Fault Tree Analysis.
- Keep causes specific and actionable.
- Document causes with supporting data (SPC charts, warranty data, field reports).
Key Takeaways #
- Failure Mode = How it fails, Cause = Why it fails.
- Causes can be design-related (DFMEA) or process-related (PFMEA).
- Identifying causes is essential for defining preventive actions.
- Causes must be specific, measurable, and actionable.
Next Lesson #
👉 Continue with Lesson 3.5.4: Cascading Relationship between Failure Cause → Failure Mode → Effect
