Taiichi Ohno
Ask why five times
Replacing the fuse, or fitting the strainer
Taiichi Ohno, Toyota Production System, 1978
The same fault keeps coming back and the review was written. What his five whys found on one stopped machine.
What actually happened?
There is an example in the book. A machine stopped. Why did it stop? An overload blew the fuse. Why the overload? The bearing was not sufficiently lubricated. Why not? The lubrication pump was not pumping enough. Why not? The pump shaft was worn and rattling. Why was it worn? No strainer had been fitted, so metal scrap got in. Only the fifth answer reaches the thing to repair: fit a strainer. Ask once and you replace the fuse, and it blows again next week. He called this the basic discipline of the Toyota floor.
The first cause is almost never the cause
A blown fuse is a fact, not a cause. It is the first link in the chain to show itself, and most reviews stop there because it is the easiest to see and the cheapest to change. Change it, and next time it is the same link again.
He was careless this time is a fuse. Keep going: why was he careless, and what made being careful impossible in that spot?
Five is not a ritual. It is the length of the chain
Fuse to strainer is exactly five steps, and each step is the answer to the why before it. Skip a level and you stop halfway, repairing something that will fail again. The number is not mystical. It is roughly how long these chains run.
In a review, take every because and ask why that, in turn, until nobody in the room can answer.
The bottom layer is usually small and cheap
A strainer costs less than a pump, less than a bearing, far less than the stoppage. Chasing a fault to its root usually ends at the smallest available action, and that is the reason to keep asking: not to be thorough, but to reach the cheap point.
A genuine root cause maps to a tiny change. If your fix is large, you have probably not reached the bottom yet.
How do I use it today?
Where you are: one kind of problem keeps recurring, you deal with it every time, you write the review, and it recurs.
Ask first: last time, did I change the fuse or fit the strainer? Counting down from the incident, how many layers did I actually reach? Was the change at that layer small?
Where it goes wrong: reading it as dig deep into everything. Five whys is for problems that repeat, and a one-off does not need it. And never let why turn into whose fault — he was tracing a chain, not a person.
Lines to keep
Repeat why five times and the real cause shows itself.
The first cause is only the first link to appear.
A genuine root cause usually maps to a very small change.
Same situation, other people are asking
We fell into the same hole again.New people, same mistakes.I have replaced the people and the same fight came back.We punished the person who did it and it happened again.The same mistakes keep happening · all 9 questions →If this one named what you are going through,
send it to someone who needs it, or keep it somewhere you will find it again.