The 5 Whys is a very simple technique but it helps you to get to the heart and root cause of a
problem. In this short video HBR describe how to use the technique and provide an example that shows that many technical problems actually have a human cause. This is a technique which I have personally used over the years with great results.
For a problem with multiple causes it can also be used in conjunction with a fishbone diagram to really help you get to the bottom of the issues.