Try it while you read
Open the free FMEA Worksheet tool and click Load sample: it reproduces the case study below.
Theory: thinking about failure before it happens
Failure Mode and Effects Analysis systematically asks, for every process step: how can it go wrong, what happens then, why would it happen and how would we notice? It is a preventive tool: the goal is to find and reduce risks before series production, not to document problems afterwards.
The three ratings
- Severity (S), 1 to 10: how serious is the effect for the customer? 9–10 means safety or legal non-compliance.
- Occurrence (O), 1 to 10: how likely is the cause, considering the prevention controls?
- Detection (D), 1 to 10: how likely is it that the failure escapes? 1 is error-proofed, 10 is no detection.
Why the action priority replaced the RPN
An RPN of 120 can mean S 10 × O 3 × D 4 (a safety risk) or S 4 × O 6 × D 5 (a cosmetic issue). The AIAG-VDA action priority table looks at the combination with severity first, then occurrence, then detection, and returns High, Medium or Low. High means action is required, Medium means action should be taken or the decision documented.
Key point
Step by step with the tool
- Add a failure mode: a card opens with three groups of fields.
- Failure: process step, failure mode, effect and severity from the list with meanings.
- Cause and current control: cause with occurrence, current controls with detection.
- Action: recommended action, owner, due date.
- Sort by risk with “Highest risk first” and work from the top.
- After the action switch on “After-action ratings” and enter the new S, O, D.
- Create the control plan draft with one click; special characteristics are proposed from severity.
Case study: process FMEA for a shaft
The Load sample FMEA has five failure modes:
| Step · failure mode | S | O | D | RPN | AP |
|---|---|---|---|---|---|
| Turning · diameter oversize | 8 | 5 | 6 | 240 | M |
| Packaging · wrong label | 6 | 4 | 7 | 168 | M |
| Heat treatment · hardness below spec | 9 | 2 | 5 | 90 | M |
| Turning · roughness too high | 7 | 3 | 4 | 84 | L |
| Assembly · circlip missing | 10 | 2 | 2 | 40 | L |
By RPN, “diameter oversize” leads. The circlip has the highest severity (10) but a low RPN because a camera already detects it (D 2) and the cause is rare: the action priority correctly rates it Low. Heat treatment has a low RPN of 90 but severity 9, so it stays at Medium and gets a continuous temperature log.
For “diameter oversize” the team adds in-process gauging every 20 parts and automatic probing after insert changes. New ratings S 8, O 2, D 3: RPN 48, AP Low.
Result
Common mistakes
- Rating detection low because “the operator checks visually”.
- Prioritising only by RPN and missing high-severity risks.
- Writing the FMEA after the process is built, just for the PPAP folder.
- Never updating the FMEA after complaints; lessons from 8D reports belong in it.
Frequently asked questions
Should I use RPN or action priority?
The AIAG-VDA FMEA handbook (2019) replaced the RPN with an action priority (AP) table because RPN treats S, O and D as equally important. Many customers now require AP. The tool shows both.
Who should take part in the FMEA?
A cross-functional team: process engineering, quality, production, maintenance, and design for design-related failure modes. An FMEA written by one person at a desk misses most of the real risks.