Action Priority (AP) vs RPN in FMEA

For decades, FMEA teams used the Risk Priority Number (RPN) to rank risks:

RPN=Severity(S)×Occurrence(O)×Detection(D)

But in practice, RPN often gave misleading results and led to inconsistent prioritization.

👉 To solve this, the AIAG-VDA FMEA Handbook (2019) introduced Action Priority (AP) as a replacement.
AP ensures that safety-critical and compliance-related risks are never ignored, even if their probability is low.

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What is RPN in FMEA? #

• Definition: RPN is a numerical value calculated by multiplying Severity, Occurrence, and Detection ratings (each on a 1–10 scale).
• Range: 1 (lowest risk) → 1000 (highest risk).
• Purpose: Originally used to sort and prioritize risks.

Example – RPN Calculation

• Severity = 9, Occurrence = 3, Detection = 5
• RPN = 9 × 3 × 5 = 135

At first glance, this looks like a “medium” risk. But if the severity = 9, it’s a safety concern that must be addressed immediately — RPN fails to highlight that.

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Problems with RPN #

1. Equal RPN for different risk profiles
   • Example: (S=9, O=3, D=5) → RPN = 135
   • Example: (S=5, O=9, D=3) → RPN = 135
     👉 The first is a safety-critical issue, the second is just frequent — but RPN treats them as equal.
2. Low RPN hides critical risks
   • A Severity = 10 with good detection (D=2) and rare occurrence (O=2) → RPN = 40 (low).
     👉 In reality, this is a life-threatening risk.
3. No universal threshold
   • Different companies used different cut-offs (e.g., “>100 = high risk”), creating inconsistency.
4. Focus on numbers, not meaning
   • Teams often chased “big RPNs” instead of focusing on safety and customer impact.

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What is Action Priority (AP) in FMEA? #

• Definition: AP is a decision-making method that assigns High (H), Medium (M), or Low (L) priorities based on combinations of S, O, and D.
• Purpose: To ensure risks are addressed according to severity and customer impact.

AP Categories

• High (H): Immediate action required.
• Medium (M): Action recommended, document decision.
• Low (L): Action may not be needed, justification required.

📌 Key Point: In AP, Severity is always the first filter — safety-critical risks (S ≥ 9) are always High Priority.

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Example – RPN vs AP Comparison #

Failure Mode	Severity	Occurrence	Detection	RPN	RPN Decision	AP Result
Airbag fails to deploy	10	2	2	40	“Low risk” (ignored)	High (H) – must act
Scratch on trim panel	3	6	5	90	“Medium risk”	Low (L) – cosmetic, no action
Engine fails to start	8	4	4	128	“Medium risk”	High (H) – loss of primary function

👉 This shows how AP corrects the weaknesses of RPN.

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Benefits of Action Priority over RPN #

1. Severity-driven: Ensures safety and compliance risks are never overlooked.
2. Clear guidance: Teams know whether action is mandatory (H), optional (M), or justifiable (L).
3. Consistency: Standardized decision-making across companies.
4. Better communication: Easier for cross-functional teams and customers to understand.
5. Focus on prevention: Shifts mindset from chasing numbers to prioritizing meaningful risks.

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Case Study – PFMEA for Bolting Process #

• Function: Secure suspension bolt at 100 ± 5 Nm.
• Failure Mode: Under-torque bolt.
• Effect: Suspension loosens → potential accident.
• Cause: Torque wrench calibration drift.
• Controls: Manual torque check.

👉 Risk Evaluation:

• Severity = 9, Occurrence = 4, Detection = 7
• RPN = 252 → “high but not urgent” in old method.
• AP = High (H) → mandatory action.

Action Taken: Introduced automatic torque monitoring system → Detection improved to 3 → AP reduced to Medium (M).

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Best Practices for AP vs RPN Transition #

• Stop using RPN in new FMEAs — follow AIAG-VDA Action Priority.
• Train cross-functional teams on H/M/L interpretation.
• Document justifications for Medium and Low ratings.
• Revisit legacy FMEAs: update critical risks with AP evaluations.
• Use AP in combination with customer-specific requirements (CSRs).

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Common Mistakes to Avoid #

• Using both AP and RPN in parallel (creates confusion).
• Lowering Severity just to get a Medium/Low AP rating.
• Overestimating detection controls (manual checks ≠ strong detection).
• Not documenting why Medium/Low AP ratings were accepted.

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Key Takeaways #

• RPN = Severity × Occurrence × Detection → outdated, misleading.
• AP = High / Medium / Low priorities → safety-driven, standardized.
• Severity always dominates AP → safety risks must be addressed first.
• AIAG-VDA requires AP evaluation for all new FMEAs.

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Next Lesson #

👉 Continue with Lesson 3.7: Step 6 – Optimization (Actions & Re-Evaluation)