FMEA is the only quality tool that works on a defect that does not exist yet. That is also why it is the one most often filled in out of obligation. I have seen two hundred line PFMEAs, every action closed and signed by the customer, in plants where scrap did not move for a year.
Usually the ratings are fine and the actions got done. The problem is which of the three ratings got attacked first, and that choice gets made almost without anyone noticing.
The three ratings, one line each
Severity is how serious the effect is for the customer. It rarely comes down without a redesign. Occurrence is how often the cause is expected. Detection is how likely you are to catch the failure before it ships.
If your customer works to the AIAG-VDA standard, priority no longer comes from multiplying them: action priority evaluates them together. Plenty of plants still use RPN. For what follows it does not matter which one you use, because the mistake shows up in both.
The easy path that looks great at audit
When a failure mode comes out high risk, the fastest action is almost always a detection action. One more inspection, a go/no-go gauge, an end of line test. It goes in within a week, the detection rating drops, and the risk turns green.
On paper the FMEA improved. And for a while the plant sees fewer defects at the customer, because it now catches them inside. That looks like a win, and that is exactly where the trap is.
What the inspection that "fixed" the risk costs
One more inspection only finds defects. The bad part still gets made, its material and machine time still get paid for, and now somebody also sorts it, reworks it or scraps it. The cost moved from the customer complaint to internal scrap.
Multiply that by every failure mode where the same path was taken and you get a plant with a spotless FMEA, an inspection station on every line, and the same scrap as always.
There is a way to read the FMEA that prevents this from the first line.
Occurrence first, detection second
Lowering detection means you will find more defects. Lowering occurrence means you will make fewer. So when I review an FMEA with a team, the first question for every high risk item is which action lowers occurrence. Only when none is possible, or while it is being implemented, does a detection action come in as containment.
Actions that lower occurrence are almost always a poka yoke or a change in process control: a pin that blocks the wrong orientation, a parameter that can no longer drift, a supplier with its own control. That is why FMEA works better tied to problem solving than as a standalone document.
And that tie has a second half almost everybody forgets.
The loop an 8D is supposed to close
Every 8D your team closes describes a failure the FMEA did not see or rated wrong. If the 8D gets filed without going back into the FMEA, the document keeps describing a process that no longer exists. The next review runs on old assumptions.
A living FMEA gets reopened when something in the process changes (supplier, tooling, shift or volume) and every time an 8D closes. If your only review date is the annual audit, the document will always lag behind reality.
Start here
Open your PFMEA and filter the actions closed in the last year. Mark which ones lowered occurrence and which ones only lowered detection. If the second column is longer, you know why scrap did not move.
I run that review with quality, process and floor teams as part of my problem solving work. If you want to do it on your FMEA, tell me which line it covers.
In an FMEA, is it better to lower occurrence or detection?
Occurrence. Lowering detection means you find more defects. Lowering occurrence means you make fewer. A detection action is useful as containment while the occurrence action gets implemented.
How often should a PFMEA be updated?
Whenever something in the process changes and every time an 8D closes. If it only gets reviewed for the annual audit, it describes a process that no longer exists.
How does FMEA relate to 8D?
An 8D describes a real failure the FMEA did not anticipate or rated wrong. When the 8D closes, the FMEA gets updated with that failure and its new rating. That closes the loop between preventing and correcting.