Most important, assume nothing. As soon as you think you know what is wrong, it is impossible not to ignore contrary evidence.

I work with some engineers who are good designers because they keep their brain fixed on the goal and work through every obstacle. They THINK they are also good troubleshooters but they are wrong, obstinately frustratingly wrong, for the same reason. They KNOW what is wrong too early, and nothing shakes their belief.

Second. When anybody tells you the symptoms, listen. When they tell you why, STOP! Don't listen. It will probably be theoretically impossible, and then you end up doubting the symptoms. Sorry but this is especially a problem with techs and mechanics. They observe correctly but theorize contrary to the laws of physics, so you need to stop listening when they get to that point.