From what you have stated that it shoots off, i would say whatever the fault, there must be a problem in the controller, as the fail safe circuit is not working. If a fault is detected in most modern controllers, it should either drop out the contactors or prevent the system from working. So it sounds like you have 2 faults one being the fault which is triggering the truck to shoot off and the other being the fail safe circuit not working. That can do some serious damage to someone get it looked at asap.