The different CAN bus systems and control units are connected in a control unit called gateway to exchange information. All the control units that are possible to program into the car are stored in the gateway control unit. When reading the gateway coding, the control units that are programed to the car are displayed as "Coded". The other control units are displayed as "Not coded". If for example a trailer control unit is installed it is necessary to change the status of this control unit in Gateway control unit to "Coded", so the other control units in the car can exchange information with the trailer control unit. It is also possible to decode control units from the gateway if a control unit is dismounted from the car.