5Why implementation theory training      Hung-Yi Liu   2014.2.12

I had no idea what today’s course was until Molex’s Kevin Yang started it. He offered it with a lot of examples to let me understand little by little. The points are as follows:


Real cause

Root cause


Corrective action (C/A)

Prevetive action (P/A)




A cause was identified in an abnormal quality issue, and it, named as real cause, can be defined as the very beginning. The cause is a realistic and practical factor.

After that, corrective action (C/A) will be applied to handle the issue.

However, 5 Why doesn’t need to be used for analyzed a real cause.

For example,

Problem: An abnormal quality occurs—dent issue discovered.

Solution: C/A can be applied to handle the problem. Metal filings in die caused the dent issue, and they were able to be cleaned with air gun, which is a problem able to be solved immediately.

After finding the real cause, we can pursue a foundational problem—so called foundational cause, able to affect the whole system, is named as root cause.

After that, P/A can be used to solve the problem. However, the root cause needs to be analyzed, improved, and set up a standard of process to prevent repetition with 5 Why.

For example,

Problem: like the dent issue previously, why the issue occurred? The real cause is metal filings.

Solution: after the real cause discovery, 5 Why can be employed to analyze its rood cause—what caused the metal filings?

Root cause


     Press machine issue

Regular maintenance

     Operation issue


Whether upper die punch and lower block were worn out

Change die punch and block material for longer life

vacuum damage caused that (metal filings) collector was not able to function well.

Replace (metal filings) collector

     Wrong type of lubricant

Find the right lubricant type

5 Why has been used to find the root cause and P/A, and a standard of process can be set up to prevent from problem repetition.


I learned from Yang’s course though it was a one-hour and condensed version.