First check that it's properly plugged in to the controller. When mine packed up, I found it was one of the three wires to the hall sensor inside the throttle had come off - required re-soldering. It had become intermittent first, which was a bit of a clue to a connectio problem. If your throttle is similar to mine, you have to remove it completely from the handle bars to get it apart. Once off, you can lever the twistgrip away from the other bit and then see inside. There may be a screw on cover to remove as well. Once you have exposed the hall sensor, you can check the continuity of each of the three wires between the connector (at the controller end) and the hall sensor with a meter. Good luck.