Stepper motors very rarely go bad. It's possible, like a bearing fails. More often the wiring goes bad, or the stepper driver, or the driver overheats.

How do I know if my stepper motor is bad?

The first sign of overheating is erratic stepper motor behavior. Typically, this can be recognized by the sounds of the stepper motor suddenly losing power (thermal shutdown). If no load or movement is required of the motor, it is hard to detect whether it is over-powered as the driver is barely producing any heat.

Why do stepper motors fail?

Short Circuit. One of the major problems with a stepper motor is complete motor failure. This problem is caused by excessive current being sent to the device by the power supply. A short circuit in the wiring from the power supply to the motor cause this problem with the stepper motor.