do steppers go bad?

my job requires repair of machines that have stepper motors. I have come across a few units that have weak steppers... I.E. they stop spinning under small load and just make a weird noise, reset and start again. I have taken the motor apart, cleaned and inspected, all looks good. i suspect the power supply or motor control circuits on the motherboard are bad... How can i diagnose my problem? There are multiple wires connecting to the stepper, can i just check for voltage? how to know which ones?

Once you take a stepper apart, it will lose most of it's power.

Is there any way to magnetize the stepper motors?

why would they go bad? doesn't make sense to me

They do go bad. I have replaced a few of them, mostly small Nema 17 steppers. The defect is similar, they begin to lose steps or resonate and stall. Once replaced everything works OK. Nevertheless, bad stepper is the last thing I suspect, it is not as frequent as bad drives.

what about over running? would this be a drive or motor problem?