During a complicated diagnosis, the most important piece of information needed could be an answer to the question “What does the system want?” It’s great to get a response from your meter or scope that tells you something is wrong, or different than what you’d expect, but if you don’t know what to expect, life can get a little complicated. Maybe you can find a diagram that gives you the power transistor wiring or something more than wires going into an empty box. Maybe you can find a flow chart that correctly explains a step by step procedure that leads you to “this part has failed.” But more often than not you won’t be so lucky.