While I completely understand the need for questioning a customer about issues to aid diagnosis there are too many that will only tell you their internet derived diagnosis rather than explaining their symptoms.
This and withholding vital information because they think that the solution will somehow cost less or to hide an error like a misfuelling makes it increasingly difficult to get the information you need to do your job and can often have the opposite effect by increasing costs such as diagnostic work because you’re missing something that would have simplified the process.
I personally believe that there needs to be some kind of campaign of confidence in technicians and not hiding information. Everyone I speak to has had similar, I personally have had vital information left out and when I have found the cause Ive approached the customer and asked, I’ve been told that it was omitted because they believed that as soon as I had heard it I’d have done XYZ and they knew XYZ was a costly repair. It may be the costly part that had failed but having the full symptoms including intermittent problems may would have resulted in the inspection of a certain component quickly minimising the diagnostic costs but instead has cost the additional tail chasing based on incomplete information.