You’ve identified how the product can fail in Step 4. Now it’s time to assess:
How critical is each failure? How likely is it to occur? Will we detect it in time?
That’s what Risk Analysis in Step 5 of AIAG-VDA DFMEA is all about.
You will now:
- Assign Severity (S), Occurrence (O), and Detection (D) ratings,
- Use Action Priority (AP) tables to guide decisions,
- Identify which failure chains need action right now.
🎯 Objective of Step 5 #
“Evaluate the risk of each failure chain using S-O-D ratings, and use Action Priority to determine where action is required.”
🚗 Case Study: Electric Water Pump (EWP) #
Let’s continue with the 12V Electric Water Pump case. Below we analyze a few selected failure chains by assigning S-O-D and calculating AP (Action Priority).
📊 1. Severity (S) Rating #
- Definition: The seriousness of the effect of the failure on the end-user or system.
- Rating Scale: 1 (no effect) to 10 (safety-critical)
Examples:
| Failure Effect | Severity (S) |
|---|---|
| Engine overheating → vehicle stops | 10 |
| Coolant leak → visible, not functional | 8 |
| Signal delay → no functional change | 4 |
✅ Use only the effect to rate Severity. Cause and detection do not affect S.
📉 2. Occurrence (O) Rating #
- Definition: Likelihood that the cause of the failure will occur in the design.
- Rating Scale: 1 (remote) to 10 (very high)
How to assess:
- Based on design maturity, testing evidence, FMEA history, simulations
- Consider prevention controls like FEA, stress analysis, margin calculations
Examples:
| Cause | Prevention Control | Occurrence (O) |
|---|---|---|
| Impeller slip | No retention feature | 6 |
| Seal wear | FMEA history + simulation | 4 |
| MOSFET overheating | Design margin + derating + simulation | 3 |
✅ Only prevention controls impact O rating.
🔍 3. Detection (D) Rating #
- Definition: Likelihood that the failure will be detected before reaching the customer.
- Rating Scale: 1 (always detected) to 10 (never detected)
Consider:
- Design verification methods (DVP&R tests, simulations, prototyping)
- Design detection controls (early warnings, diagnostics, sensors)
Examples:
| Detection Method | Timing | Detection (D) |
|---|---|---|
| Leak test @ DV only | Late | 6 |
| Thermal simulation + bench test | Medium | 4 |
| EMC pre-compliance scan | Early | 3 |
✅ Only detection controls impact D rating.
📊 4. Action Priority (AP) #
The AIAG-VDA manual replaces RPN (Risk Priority Number) with a smarter approach: Action Priority (AP).
🔺 Action Priority Levels:
- High (H): Must take action or justify why not
- Medium (M): Consider action; justify if none taken
- Low (L): No action required
You determine AP using a lookup table combining S, O, and D.
📥 Download AP lookup table for AIAG-VDA (PDF/Excel)
📋 EWP DFMEA Risk Analysis Examples #
| Function | Failure Mode | S | O | D | AP | Reasoning |
|---|---|---|---|---|---|---|
| Deliver coolant | No flow | 10 | 6 | 5 | H | High severity, medium likelihood, poor detection |
| Prevent leak | Leakage | 8 | 5 | 5 | H | Regulatory impact, moderate detectability |
| EMC compliance | Emission fail | 9 | 3 | 4 | M | Pre-scan exists but not full test |
| Signal control | PWM delay | 4 | 2 | 3 | L | Low impact, low O/D |
✅ These AP values help you prioritize actions in Step 6 – Optimization.
📑 Template: DFMEA Risk Analysis Worksheet (Excerpt) #
| Function | FM | Effect | Cause | S | O | D | AP | Action Needed? |
|---|---|---|---|---|---|---|---|---|
| Deliver flow | No flow | Overheat | Impeller slip | 10 | 6 | 5 | H | Yes |
| Prevent leak | Leak | Loss of coolant | Seal wear | 8 | 5 | 5 | H | Yes |
| EMC control | Emission > limit | ECU interference | No shielding | 9 | 3 | 4 | M | Evaluate |
| PCB control | Delay | Late motor start | PWM logic lag | 4 | 2 | 3 | L | No |
📥 Add this as an Excel template to your downloadable files.
✅ Outputs of Step 5 #
By the end of this step, you will have:
- ✅ Assigned S-O-D ratings for each failure chain
- ✅ Calculated Action Priority (AP) per AIAG-VDA lookup
- ✅ Identified which failures need immediate action
- ✅ Prepared your DFMEA for Step 6: Optimization
🔗 Internal Linking Suggestions #
- Step 6: Optimization – Reduce Occurrence & Detection
- Download: AP Lookup Table (PDF)
- Download: Risk Rating Worksheet Template (Excel)
🧠 Pro Tip #
“Let the AP guide your focus. Don’t waste time fixing Low AP issues while High AP ones stay unresolved.”
🏁 Conclusion #
Risk Analysis transforms your DFMEA from a list of problems into a data-driven risk map. You now know:
- What’s severe,
- What’s likely,
- What’s hard to detect,
- And what needs fixing first.
You’re ready to move into Step 6 – Optimization, where we improve your design to reduce risk and close the loop on actions.
