I've had nothing but great support from tekin. If they can't find anything wrong with it then there probably isn't. They wouldn't have just sent out back to you without checking and testing it. As the previous poster says run it again and see what happens. The cogging sensation could be down to a sensor issue as well are you using a new sensor lead? You can also check if the esc is seeing the sensors. With the esc all powered up manually rotate the wheels and watch for the 3 leds on the esc dimly flashing - it's a great feature of the esc!