Critical Characteristics in FMEA: What to Do When Severity Hits 9 or 10

Finding a severity 9 or 10 in your FMEA is where the paperwork exercise ends and the real work begins. Most FMEA training covers how to assign severity ratings; fewer resources cover what happens next—the specific required actions, the control plan documentation, the Cpk targets, and the evidence auditors actually check. This covers what quality engineers must do when the severity column turns critical.

What Severity 9 and 10 Mean in Practice

In the AIAG-VDA methodology, severity 9 indicates that the failure mode could result in regulatory non-compliance or injury to the operator or vehicle occupant. Severity 10 indicates that the failure mode occurs without warning and could result in loss of life or serious injury. Both levels designate the associated characteristic as a Critical Characteristic (CC).

The distinction between 9 and 10 matters for two reasons. First, severity 10 items typically require design-level mitigation—the risk cannot be managed solely through process controls. Second, many customers and OEMs require escalated notification for severity 10 items during PPAP. Lumping 9 and 10 together in your risk assessment can mask this difference.

Severity 5–8 items designate Significant Characteristics (SC). These require attention but not the same mandatory escalation path. The SC designation uses a diamond symbol in FMEA documentation; the CC designation uses an inverted triangle. These symbols flow from the FMEA into the control plan, process flow diagram, and drawing—consistently, not just in the FMEA column.

The Key Constraint: Severity Can Only Be Reduced Through Design Change

Adding a detection control—an inspection station, an in-process measurement, a vision system—does not reduce severity. If a failure mode causes injury or regulatory non-compliance when it reaches the customer, better detection at the end of the line reduces your detection rating but leaves severity unchanged. This is one of the most common misconceptions in FMEA reviews, and it leads to fatally flawed risk assessments.

The only way to reduce a severity 9 or 10 rating is to change the design so that the failure mode either cannot occur or cannot produce that level of consequence. In practice, this means:

• Redesigning the component or assembly to eliminate the failure mode entirely
• Adding a physical safeguard that prevents the failure from propagating to the safety effect (a secondary retention feature, a pressure relief valve, a mechanical interlock)
• Changing the functional specification so that the effect of the failure is bounded below severity 9

If none of these are possible—and sometimes they aren’t, especially on late-stage PFMEA items where the design is fixed—then the severity 9–10 item stays where it is. The FMEA documents that the risk is understood and managed through occurrence and detection controls, even if severity itself remains high. The FMEA severity rating scale definitions cover the full 1–10 scale criteria with manufacturing and assembly examples.

Required Actions for Critical Characteristics

Once a CC is identified, four things must happen before PPAP:

1. Classification column marked in the FMEA. The AIAG-VDA format includes a Classification column specifically for SC/CC designations. It must be populated—not left blank—for every failure chain that produces a severity 9 or 10 effect. Auditors look here first when reviewing FMEA coverage of customer-designated safety characteristics.

2. Control plan entries with appropriate controls. Every CC must appear on the control plan with specific measurement methods, sample frequencies, and reaction plans. The control method must be capable of detecting the CC before the part ships. “Visual inspection 100%” typically does not satisfy CC requirements—objective measurement is expected. Gauge type, measurement resolution, and measurement frequency should match the criticality of the characteristic. See the post on keeping the PFD-PFMEA-control plan trio in sync for how this linkage is maintained across documents.

3. Process capability targets met. The AIAG core tools specify Cpk ≥ 1.67 for critical characteristics during PPAP validation. This is above the 1.33 minimum required for non-critical capable processes. If your initial capability studies show Cpk below 1.67, the FMEA action plan must include process improvement actions to close the gap before production approval. Documenting “Cpk = 1.25, CC” without a corrective action plan is an audit finding.

4. Customer notification and approval. Most OEM customers require notification when new critical characteristics are identified, particularly if they weren’t designated in the original customer drawing. Automotive OEMs often require customer engineering sign-off before the CC designation is finalized. Skipping this step creates risk in both directions: the customer may dispute the designation at PPAP, or may later argue they weren’t informed of a safety risk.

What the Action Priority Table Does with Severity 9–10

Under the AIAG-VDA Action Priority methodology, any failure chain with severity 9 or 10 automatically receives an Action Priority of High (H) regardless of occurrence or detection ratings. This is a deliberate design of the AP system—it corrects for the most dangerous flaw in RPN-based risk management, where S=10, O=1, D=1 produces RPN=10 (appears low risk) despite representing a catastrophic failure that can sometimes occur and is never detected.

Action Priority H means: action is required. The FMEA cannot be closed out with the item in the High category without documented recommended actions and a completion timeline. A team that documents “AP=H, no action planned” is creating liability, not managing risk.

If your organization still uses RPN, apply a severity gate independently of the RPN threshold. RPN thresholds (e.g., “action required if RPN > 100”) will systematically miss severity 9–10 items with low occurrence and detection. Many quality systems use RPN as a triage tool but mandate action on all severity 9–10 items regardless of RPN score.

Documenting Evidence for Audits

When an IATF 16949 auditor or OEM customer auditor reviews your FMEA for critical characteristics, they look for a specific evidence chain:

1. FMEA classification column — CC or inverted triangle symbol present in the row
2. Control plan linkage — The same characteristic appears on the control plan with a matching CC designation and appropriate controls
3. Capability data — Initial capability studies (Ppk during PPAP, Cpk after 90–180 days of production) meeting the 1.67 threshold
4. Recommended action evidence — If actions were assigned to reduce occurrence or detection on the CC item, documented completion evidence with before/after ratings
5. Customer notification record — Written evidence that the CC was communicated and approved by the customer if required by the customer-specific requirements

Missing any link in this chain is a finding. The most common gaps are: CC in FMEA but not transferred to control plan, CC on control plan but no capability data, and recommended actions closed without documented evidence of completion. For more on how to document and close recommended actions, see the post on prioritizing FMEA actions when resources are limited.

When All Severity 9–10 Items Are the Same Characteristic

A pattern that comes up in complex assemblies: the FMEA has twelve severity 9–10 rows, but eleven of them trace to the same physical characteristic—say, a bolt torque that, when outside spec, can cause loss of structural retention. This is not twelve separate critical characteristics. It’s one characteristic (bolt torque) that appears in twelve places in the FMEA because the failure analysis was done at multiple structure levels.

When this happens, consolidate the CC designation onto the single characteristic in the control plan. Twelve control plan rows for the same bolt torque does not improve quality management—it creates confusion about where the authoritative control specification lives. The FMEA can retain all twelve rows for analytical completeness, but the control plan and drawing should designate the characteristic once.

The RPN and action priority calculator automatically flags severity 9–10 entries as AP High and highlights them for action review, which helps quality engineers identify all CC items in a batch before moving to control plan updates.