FMEA: RPN vs Action Priority

Short answer: both are ways to decide which failure modes to work on first. The RPN multiplies three ratings: Severity × Occurrence × Detection, each scored 1–10. Action Priority (AP), used in the newer AIAG-VDA FMEA approach, puts each S-O-D combination into High, Medium or Low priority with a lookup table that gives the most weight to severity, then occurrence, then detection.

Why RPN can mislead

Multiplication treats the three ratings as equally important, and very different risks can get the same score. A safety failure that rarely happens can score lower than a cosmetic defect that happens often.

Worked example

What it means: rank by RPN and you'd fix the label first. Almost any team would want the brake fitting reviewed first, though, because a severity of 9 means a safety or regulatory effect. Severity-first logic like AP puts A ahead of B. Whichever method your organization uses, also apply a severity rule: any 9 or 10 gets a look no matter what the other ratings are.

How to lower the risk

• Severity usually only changes if you change the design (for example by removing the failure mode or adding redundancy).
• Occurrence drops when you prevent the cause: error-proofing, better process control, design margin.
• Detection improves with better controls, but detecting a failure doesn't stop it from happening. Prevention beats detection.

Common exam traps

• Using a fixed RPN threshold (such as "act on everything over 100") as if it were a rule. Ratings are ordinal, so RPN cutoffs are arbitrary.
• Detection scale direction. A high detection number means the failure is hard to detect.
• Confusing failure mode, effect and cause. The mode is how it fails, the effect is what the customer experiences, and the cause is why it happens.
• Treating FMEA as one-and-done. Re-rate after actions, and update the FMEA when the design or process changes.

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