In DFMEA, once we’ve defined what the system must do (Step 3 – Function Analysis), we now ask:
“What happens if it doesn’t?”
That’s the core question of Failure Analysis, Step 4 of the AIAG-VDA DFMEA 7-Step Approach.
This step lays the risk foundation of your DFMEA. You will now:
- Identify how each function can fail,
- Analyze the effects on next-higher level and end-user,
- Trace the causes at next-lower level,
- Start thinking about Severity (S) ratings.
🎯 Objective of Step 4 #
“Identify the failure modes, their effects at system and customer levels, and the causes at component level. These build the base for risk evaluation.”
🔄 Failure Chain Concept (S → O → D) #
DFMEA Failure Chains typically follow this flow:
Cause (next-lower level)
⬇
Failure Mode (element itself fails to perform function)
⬇
Effect (next-higher level + vehicle/user impact)
⬇
Severity (S) → Evaluated in next step
🚗 Case Study: Failure Analysis for 12V Electric Water Pump (EWP) #
We’ll now walk through several real-world examples from the Electric Water Pump (EWP) DFMEA.
📋 Sample Failure Chain 1: “No coolant flow”
| Level | DFMEA Entry |
|---|---|
| Function | Deliver coolant flow (EWP) |
| Failure Mode | No or insufficient coolant flow |
| Effect – Next Higher | Engine not cooled properly |
| Effect – End User | Engine overheating, vehicle breakdown |
| Severity | 🔥 S = 10 (safety impact) |
| Cause(s) | Impeller slip on shaft, rotor demagnetization, blocked coolant path |
📋 Sample Failure Chain 2: “Coolant leakage”
| Level | DFMEA Entry |
|---|---|
| Function | Prevent leakage at rotating seal |
| Failure Mode | Visible or internal leak |
| Effect – Next Higher | Coolant loss, corrosion of PCB |
| Effect – End User | Vehicle warning light, risk of failure |
| Severity | ⚠️ S = 8 (vehicle function impact) |
| Cause(s) | Seal wear, shaft runout, improper material, installation damage |
📋 Sample Failure Chain 3: “High EMC emissions”
| Level | DFMEA Entry |
|---|---|
| Function | Maintain EMC compliance (PCB) |
| Failure Mode | Radiated emissions exceed limits |
| Effect – Next Higher | Disturbance in ECU communication |
| Effect – End User | Erratic cooling behavior |
| Severity | ⚠️ S = 9 (functional, regulatory impact) |
| Cause(s) | Missing choke, poor layout, bad ground stitching, no shielding |
🧠 Tips for Writing Good Failure Chains #
| Element | Best Practice |
|---|---|
| Function | Use active verb + measurable output (“Rotate at 4000 RPM”) |
| Failure Mode | Use clear deviation from function (“No flow,” “Exceeds limit”) |
| Effects | Separate technical effect and customer consequence |
| Causes | Keep technical, specific, and component-level |
| Avoid | Vague entries like “not working” or “design issue” |
🧩 Common Failure Modes in EWP DFMEA #
| Component | Function | Common Failure Modes |
|---|---|---|
| Impeller | Generate flow | Slip, break, cavitation erosion |
| Shaft | Transmit torque | Misalignment, fatigue crack |
| Seal | Prevent leak | Hardening, chemical attack, wear |
| PCB | Control motor | Overheat, component damage |
| Connector | Provide electrical link | Pin bend, contact resistance ↑ |
| Housing | Contain pressure | Crack, porosity, distortion |
🔁 Repeat Failure Chains for All Key Functions #
You should repeat this process for:
- Each function in your Function Analysis table
- Each interface (they often lead to severe failures)
- Special characteristics (must analyze thoroughly)
📑 Template: Failure Chain Worksheet (Excerpt) #
| Item | Function | Failure Mode | Effect (System) | Effect (User) | Cause | S |
|---|---|---|---|---|---|---|
| EWP | Deliver flow | No flow | Overheat risk | Engine damage | Impeller slip | 10 |
| Seal | Prevent leak | Leakage | Internal corrosion | System failure | Shaft runout | 8 |
| PCB | Control motor | Overcurrent | Signal loss | Pump stops | MOSFET failure | 9 |
✅ These entries will now transition into the full DFMEA form for Step 5 – Risk Analysis.
🧪 Functional Failure Chain Diagram (Optional Visual Aid) #
Cause (Shaft runout)
↓
Failure Mode (Seal leaks)
↓
System Effect (Coolant ingress into PCB)
↓
User Effect (Pump failure, warning light)
↓
Severity = 8
📥 Add this as a visual template in downloads
✅ Outputs of Step 4 #
By completing Step 4, you will have:
- ✅ Defined all failure modes for your product functions
- ✅ Linked each with system-level and customer-level effects
- ✅ Identified specific causes for each failure
- ✅ Assigned initial Severity (S) ratings (Step 5 will complete risk evaluation)
- ✅ Prepared complete failure chains for DFMEA rows
🔗 Internal Linking Suggestions #
- Step 5: Risk Analysis – Severity, Occurrence, Detection & AP
- Download: DFMEA Failure Chain Template (Excel)
- Guide: How to Write Effective Failure Modes
🧠 Pro Tip #
“If the failure mode is vague, your risk ratings will be meaningless. Clarity here equals accuracy later.”
🏁 Conclusion #
Failure Analysis is where DFMEA becomes real. You’re no longer theorizing — you’re diagnosing how things break, what happens next, and what’s truly critical.
In the next step — Step 5: Risk Analysis — we will quantify risk using S-O-D ratings and Action Priority (AP) decisions.
