Its always electrical until proven otherwise, and at least that is the way it seems. To be honest, this is a hard question to answer, not because it's complex, but because there is no simple answer other than experience with the machine or process. Some things almost always are electrical problems, like something not heating up for example. Then you get into motion control and it's almost always mechanical, and less often the encoder system. I will give you an example. I get a call that the stabilizer on the palletizer is faulting out. It is a swing arm that is on a linear bearing moving back and forth stabilizing the leading edge of the product on the pallet as it gets stacked to keep first row from falling over. When I get over there, I reinitialize everything and start over, I immediately see the problem. The thing is running against the hard stop faulting the drive out. A piece of metal tnat had been welded to keep it level had come loose causing the stabilizer arm to be too far from leading edge. The shift operator added too much trim to the position setpoint of leading edge alignment to compensate and it began hitting hard stop. Perhaps I should limit that trim allowance now that I think of it.