An on-board unit (OBU) coupled to an integrated starter generator (ISG) is disclosed. The ISG controls an engine. The OBU includes a communication unit and a processing unit. The communication unit receives a control signal from a mobile device. The processing unit is coupled to the communication unit for setting a setting state of the ISG according to the control signal, wherein the setting state is a start unallowable state or a start allowable state, and the ISG forbids the engine to be started or allows the engine to be started according to whether the setting state is the start unallowable state or the start allowable state. The control signal generates the setting state according to a start counter value, wherein the setting state is set to the start allowable state or the start unallowable state according to whether the start counter value is larger than a predetermined value.