OnStar will not stop the car (can't slow it down AFAIK) unless they are told to do so by the Police. I owned a car rental company and tried to do this with OnStar a few times but they would not. Even more frustrating, the Police can take a long time to respond to any car theft unless the thieves are still there. Police need to interview the victim before they do anything including calling OnStar. I had quite a few cars stolen and Police treat auto theft as a low priority. They assume insurance will look after the victim, so other urgent calls take priority. Also if the vehicle is in a no cell signal area, or if it is already in a shipping container, it does not work. The only unique OnStar benefits are if you crash and are unconscious they will dispatch ambulance and they will unlock the door if you somehow locked the key in the trunk. OnStar is a waste of money IMO.