What a PFMEA line contains
- Process step and function: for example, press bushing into housing.
- Failure mode: what goes wrong at that step (bushing not fully seated).
- Effect: what the next process or the end customer sees (bushing works loose, noise, loss of function).
- Cause: why the step can fail (worn locating pin lets the housing tilt).
- Prevention controls: what stops the cause from happening.
- Detection controls: what finds the failure mode or cause before the part leaves.
- Ratings and actions: S, O, D, then RPN or Action Priority, with owner and due date.
Severity, occurrence, detection
Each rating is scored from 1 to 10. Use the tables from the FMEA handbook your customer requires; the short descriptions below are a plain-language summary, not the official tables.
- Severity (S): how bad the effect is for the customer or end user. High values are for safety or legal effects. Severity belongs to the effect, so process actions do not normally change it.
- Occurrence (O): how likely the cause is, given the prevention controls in place. Better prevention (error-proofing, a fixed tool design) lowers O.
- Detection (D): how likely the current detection controls are to catch the failure before the part ships. Low values mean detection is very likely. A visual check by an operator rates worse than an automatic check that stops the line.
RPN and Action Priority
The classic risk priority number is RPN = S × O × D, from 1 to 1000. It is easy to compute but it treats the three ratings as equal. Two fictional lines show the problem:
| Line | S | O | D | RPN |
|---|---|---|---|---|
| Safety effect, rare, well detected | 9 | 2 | 2 | 36 |
| Cosmetic mark, frequent, poorly detected | 3 | 6 | 6 | 108 |
By RPN the cosmetic line ranks first, although a team would normally review the safety line first. Action Priority, used in the AIAG-VDA FMEA Handbook, avoids the multiplication: it looks up the S, O, D combination in a table and gives high, medium or low priority, with severity given the most weight. RPN is shown here only to keep the arithmetic visible; if your customer requires AP, take the S, O, D combination to the handbook's table. Whichever method you use, also review every high-severity line on its own.
Worked example: press-fit bushing
A customer reports bushings working loose (fictional). D4 finds a worn locating pin that lets the housing tilt in the press (occurrence cause), and a visual check that cannot see a partly seated bushing (escape cause). The PFMEA line before and after D7:
| Prevention / detection | S | O | D | RPN | |
|---|---|---|---|---|---|
| Before | Pin checked at yearly maintenance / visual check by operator | 7 | 6 | 6 | 252 |
| After D7 | Hardened pin, wear check in daily start-up checklist / press force and stroke monitoring on every part, line stops on fail | 7 | 3 | 3 | 63 |
Severity stays at 7 because the effect on the customer has not changed. O drops only after D6 shows the pin change works on production parts. D drops only after known-bad or seeded parts have been run through the force monitoring and stopped the line. The ratings here are illustrative; rate your own line with your customer's tables.
How the 8D and the PFMEA link
- D4: check whether the failure mode and both causes (why it happened, why it escaped) are in the PFMEA. If not, that gap is part of the root cause story. See escape point.
- D5/D6: the corrective actions become new prevention and detection controls; D6 data justifies new O and D ratings. See D6 verification.
- D7: update the PFMEA (new revision, changed rows), the control plan and the work instruction, and read across to similar processes. See D7 prevent recurrence and the PFMEA and control plan update checklist.
- D8: attach the revised PFMEA and control plan pages, or their revision numbers, as closure evidence.
PFMEA gaps a reviewer may point out
- The cause column says “operator error”. See why operator error is not a root cause.
- O rated low with no data behind it, even after a complaint on that line.
- Detection rated as if an inspection exists that is not in the control plan.
- Severity lowered after a process change.
- PFMEA updated, control plan and work instruction not.
FAQ
- What is a PFMEA?
- A process FMEA lists, step by step, how a manufacturing or assembly process could produce a bad part (failure mode), what that would do to the customer (effect), why it could happen (cause), and which controls prevent or detect it. Each line gets ratings for severity, occurrence and detection, and the riskiest lines get actions.
- Should I use RPN or Action Priority?
- Use the method your customer requires. Some automotive customers ask for Action Priority (AP) from the AIAG-VDA FMEA Handbook, which ranks combinations of S, O and D as high, medium or low instead of multiplying them. Others still accept RPN. Check the customer's supplier manual and use the rating tables from the handbook you actually follow; do not mix scales.
- Can I lower occurrence or detection when I close the 8D?
- Only when the D6 data supports it. A planned action is not yet a reason to lower a rating. Severity normally stays the same unless the design or the product function changes.
- Why was the complaint not in our PFMEA?
- Possible reasons: the failure mode was never listed, the cause was listed with a different wording, or occurrence was rated low without data. Each of those is worth one line in D4 and a PFMEA change in D7.
- Is the control plan the same as the PFMEA?
- No. The PFMEA is the risk analysis; the control plan is the list of checks that run in production (characteristic, method, sample size, frequency, reaction plan). A new detection control in the PFMEA should appear in the control plan and in the work instruction the operator uses.