Test failure Root cause not found

It's not a bad thing to tell a customer that you've exhausted all of the possibilities and are unable to duplicate the failure. The important thing is to explain in detail what you've done in trying to identify the cause. Again, it's important to understand the customer's test method(s) and compare them to your own. Was the product DOA, or did it fail in burn-in testing, for example. Do the best you can, then tell the truth.

One thing is not clear for me:
- You can confirm the claimed failure type, but cannot figure out the root cause, OR
- Failure doesn't represent on the claimed goods? (No Trouble Found)

We are also facing issues like that. We spend a lot of time with analysis, and cannot find out the root cause 100%. We always have theories.
When if it is so, I present the truth to the customer, and tell them, roo cause cannot be defined clearly. Show them an Ishikawa (or whatever tool you are using) about the options, and that's it.
Sometimes they keep pushing, and telling me "But which is the real / final root cause?" In this case I tell them funnily that "Chose one ".

Make actions for these root cause possibilities and you should be fine I guess.

D1124 said:

Make actions for these root cause possibilities and you should be fine I guess.

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When a “real” single root cause cannot be established and you end up with multiple potential causes, you may determine the implementation of “preventive actions” and the customer should be happy as long as the problem does not reoccur.

Some customers act as though you should be able to determine a root cause every time for every event. But it is not always possible. When possible, it is not always practical.