After identifying functions, requirements, failure modes, effects, and causes in Steps 3 and 4, the next step in the AIAG-VDA 7-Step FMEA is Risk Analysis.
👉 Risk Analysis answers the question:
“How big is the risk, and what should we do about it?”
In the past, FMEAs used the Risk Priority Number (RPN) to rank risks, but the AIAG-VDA standard replaced this with Action Priority (AP), which is more reliable and safety-focused.
Objectives of FMEA Risk Analysis #
- Assign Severity (S) to each failure effect.
- Assign Occurrence (O) to each failure cause.
- Assign Detection (D) to each failure control.
- Use the Action Priority (AP) table to decide which risks require action.
1. Severity (S) – How Serious is the Effect? #
- Severity measures the impact of a failure effect on the end user or system.
- Scale: 1 = no effect → 10 = hazardous, safety-critical effect.
📌 Example – Airbag DFMEA
- Effect: Airbag fails to deploy → Severity = 10 (safety risk).
👉 Severity is always linked to effects.
2. Occurrence (O) – How Likely is the Cause? #
- Occurrence measures the probability of the cause happening.
- Scale: 1 = remote chance → 10 = very high frequency.
- Based on SPC data, field failures, or expert judgment.
📌 Example – PFMEA Welding
- Cause: Electrode wear.
- Historical data: 2 defects per 1000 parts → Occurrence = 4 (low).
👉 Occurrence is always linked to causes.
3. Detection (D) – How Likely is it to be Caught? #
- Detection measures the ability of controls to detect the cause or mode before failure reaches the customer.
- Scale: 1 = almost certain detection → 10 = no detection possible.
📌 Example – PFMEA Bolting
- Control: Torque wrench calibration check monthly.
- Detection effectiveness: Moderate → Detection = 6.
👉 Detection is always linked to controls.
4. Action Priority (AP) – Deciding What to Do #
Instead of multiplying S × O × D (old RPN method), the AIAG-VDA FMEA Handbook introduced Action Priority (AP).
- AP uses a decision table that combines S, O, and D.
- AP categories:
- High (H): Immediate action required.
- Medium (M): Action recommended.
- Low (L): No action required (document justification).
📌 Example – PFMEA Bolting Process
- Severity = 9 (safety-critical), Occurrence = 3, Detection = 6.
- RPN (old) = 162 → looked medium risk.
- AP (new) = High Priority (H) → must act.
👉 AP ensures safety-critical risks are never ignored, even if occurrence is low.
Why Action Priority is Better than RPN #
- Severity-driven: High-severity failures (S=9–10) always demand attention.
- Standardized: Removes company-to-company differences in RPN thresholds.
- Clear Actions: Instead of chasing numbers, teams follow H, M, L categories.
- Customer-focused: Aligns with OEM requirements for safety and compliance.
Case Study – Risk Analysis for Welding PFMEA #
- Function: Weld two steel sheets.
- Requirement: ≥ 5 kN strength.
- Failure Mode: Weak weld.
- Effect: Vehicle structural failure in crash.
- Cause: Electrode wear.
- Controls: Visual inspection of weld nugget.
👉 Risk Evaluation:
- Severity = 10 (safety).
- Occurrence = 4 (low).
- Detection = 8 (unlikely with visual check).
- AP Result = High Priority (H).
Action: Team adds automatic electrode monitoring system, improving Detection to 3.
Best Practices for Risk Analysis #
- Always rate Severity first, then Occurrence, then Detection.
- Use cross-functional teams to avoid bias in ratings.
- Base ratings on data (SPC, warranty, lessons learned), not assumptions.
- Focus on AP categories (H, M, L) instead of chasing RPN numbers.
- Document justifications for Medium or Low AP decisions.
Common Mistakes to Avoid #
- Using RPN instead of AP in new FMEAs.
- Confusing occurrence with detection.
- Ignoring high-severity risks just because occurrence is low.
- Copy-pasting ratings from old FMEAs without validation.
Key Takeaways #
- Risk Analysis = Severity + Occurrence + Detection → Action Priority (AP).
- Severity → Effect, Occurrence → Cause, Detection → Controls.
- Action Priority (H, M, L) ensures consistent, safety-focused decisions.
- This step transforms FMEA from a document into a risk prevention tool.
Next Lesson #
👉 Continue with Lesson 3.6.1: Severity, Occurrence, Detection Scales (AIAG-VDA)
