Simple enough, I believe, to avoid much of the initial confusion. Our senses or instruments are the primary source of such indication. Failure symptoms may or may not tell us exactly where the pending failure is located or how close to the full failure condition we might be. Once the failure has occurred, any indication of its presence is no longer a symptom we now observe its effect. It is extremely important for us to understand this simple definition because, in the maintenance world, it is the failure mode that we try to prevent, or, failing that, what we have to physically fix.
Failure cause is often very difficult to fully diagnose or hypothesize.
Even though we may know the cause, we may not be able to totally prevent the failure mode or it may cost too much to pursue such a path. Thus, PM tasks cannot fix the cause they can address only the mode. This is an important distinction to make, and many people do not clearly understand this distinction. To be complete, this is usually done at three levels of assembly local, system, and plant. In describing the effect in this fashion, we clearly see the buildup of the consequences.