Quality managementReading time 13 min

The new FMEA dropped RPN and uses AP instead

RPN scores severity 9, occurrence 1, detection 1 as a 9, the same as severity 1, occurrence 3, detection 3. Those two things are obviously not equally important.

01Three ratings

Item	Asks	A score of 1	A score of 10
Severity S	How serious the consequence is when the failure occurs	Almost no effect	Affects safety or violates regulation, without warning
Occurrence O	How often this cause happens	Practically impossible	Very high, happening continuously
Detection D	How easily current controls catch it	Almost certain to be caught	Cannot be detected, or is never checked

Note that D runs the other way: a higher score means less likely to be caught.

02What is wrong with RPN

The old edition ranked by RPN = S × O × D, with a threshold usually set at 100 or 125. There are three problems.

The same product, different meaning. 9×2×2 = 36 and 2×9×2 = 36 score identically, but the first is a safety issue and the second a common cosmetic flaw.
It is discontinuous. Of 1000 possible products only 120 are distinct values, so a great many items tie.
It invites gaming. To get under the threshold, change D from 7 to 6 and it passes, which changes the number, not the risk.

03Action Priority

The 2019 AIAG-VDA edition uses Action Priority, read from a table as H, M or L, and explicitly looks at S first, then O, then D.

Situation	AP	Meaning
S 9-10 with O 4-10	H	Action must be taken, or a reason given for not taking it
S 9-10 with O 2-3 and high D	H	As above
S 7-8 with O 6-10	H	High priority
Mid-range combinations	M	Action should be taken, or the rationale recorded
Low S with low O	L	Action may be omitted, but record it

The AP table is looked up, not multiplied, that is the essential difference.

H does not mean “must be reduced to L”. It means action must be taken, or the reason for not acting formally recorded. That change turns FMEA from scoring back into deciding.

04The seven steps

Planning and preparation. Define the scope, project boundaries and participants (the 5T: intent, timing, team, task, tool).
Structure analysis. Break the system into three levels: system, subsystem, and component or process step.
Function analysis. What each level has to do, expressed so it can be measured.
Failure analysis. The chain of failure effect, failure mode and failure cause.
Risk analysis. Rate S, O and D and look up the AP.
Optimisation. Propose actions for H and M items, assign owners and dates, and re-rate.
Documenting results. Communication and records, internal and external.

05An FMEA is a living document

The most common failure of an FMEA is not that it was written badly. It is that nothing ever changed it again. All of the following should trigger an update:

a customer complaint or a serious internal defect (D7 of an 8D should go back and change the FMEA)
a design change, material change or supplier change
a change of process parameters or equipment
a clear gap between the actual early-production defect rate and the O originally rated