The check control system is part of the vehicle electronics and, after ignition and during driving operation, it monitors various vehicle functions such as the lighting functions parking lights, dipped beam, full beam, braking lights, number plate light, reverse lights and fog lights. If the lighting fails, this is shown in the dashboard via light symbols, audible signals or in the multi-function display of the on-board computer (sometimes as text). As there are vehicles both with and without check control, expansion kits are available in the wiring harness range. These can then be used as an expansion to the base wiring harness (7-pin or 13-pin). The check control expansion kit ensures that vehicle function is fulfilled with high-resistance and loss-free, and it is switched via permanent positive supply (terminal 30) on the trailer.