FX3 decoders have motor isolation protection. If the decoder senses that the motor is not isolated, it will not run the motor. In this case, you will be able to control the loco’s functions but the motor will not work.
FX decoders will blink the loco's lights when power is first applied to the decoder to warn you of a short circuit in your installation. If you see the lights blinking, immediately remove the loco from the track, locate the short circuit and correct the problem before proceeding. Do not leave the loco on the track with blinking lights because eventually, the decoder will become overloaded and be damaged. If a short occurs after initial power up, the lights will not blink so, it is important to secure the leads inside the loco once the installation is complete to prevent them from coming loose during normal operation and creating a short. For board decoders, it is important to follow the instructions for installing insulating tape inside the loco to prevent shorts during operation caused by the decoder board shifting inside the engine.