You can pull the suspect motor and hook it up to another ESC and receiver to see if it spins. If the motor spins, the 4 in 1 is shot. If the motor doesn't run, motor's bad. WARNING- If the motor doesn't spin, shut down the throttle quickly or you can kill your test ESC. If you don't have the spare parts you can disconnect the wires to the bad motor and test to see if you're getting current to the motor. The circuit testers are cheap and simple, gator clip on one end and a "pen" point on the other with a light in between. Disconnect your wires from the bad motor, hook up the test lightto the wires from the 4 in 1, install the battery, throttle up. If your test light comes on, the motor is bad. No light means the 4 in 1 is bad. Have fun!