Dirty or Damaged Motor(s)
The motor has strong magnets that can pick small pieces of metal. Staples, washers, screws and other debris can be easily sucked up through one of the motor's cooling vents, which can prevent the armature from spinning. Remove the motor from the vehicle or water craft, and try to spin the pinion gear.
If the pinion gear will not move, inspect the motor for small particles inside the motor can. Spray electric motor cleaner into the motor to remove dirt and debris and re-oil the bushings with a quality bushing oil. If a thorough cleaning doesn't work, replace the motor.
You can test a brushed motor by applying direct voltage to the +/- motor terminals. Unplug the red and black motor leads from the speed control and touch them directly to a battery's red and black wires. This direct voltage should rotate the motor at high rpm. If testing the motor while in the vehicle, be sure it is on a stand and your hands are not in the way of props or tires. If the motor does not turn quickly when voltage is applied, the motor will need to be replaced.
To remove the motor from your two-wheel drive model begin by removing the screws that hold the gear cover to the gear box. Remove the pinion gear from the motor shaft. Next, remove the two screws that hold the motor to the gear box. Now that the motor is free from the gearbox it will be easier to unplug the bullet connectors. To unplug the bullet connectors grasp the end coming from the ESC in one hand and the end from the motor in the other hand. The bullet connectors have a firm connection and may require some force when separating. You could also grasp both wires from the ESC in one hand and hold the motor in the other hand and pull the plugs apart.