The motor can be sold in one complete assembly with the field and transmission (such as in many cordless drills) or sold separately from the field and various gears. The most common reason motor go bad is due to overuse; prolonged use will overheat the motor. If the motor has a separate armature, inspect the commutator (where the brushes contact) and confirm no sections are badly burnt or missing. If you hear grinding, most likely one or more gears have sheared and need to be replaced.
This item fits the following models:
3-Nailer Combo Kit w/ Accessories
3-Nailer Combo Kit
1/4" x 50 Ft Noodle Air Hose with Coupler and Plug