Abstract: A vehicle security system with an anti-theft controller which conducts a data exchange with at least one function controller, particularly an engine controller, in order to control its operational readiness, wherein the data exchange is conducted via a data line connecting at least the anti-theft controller and the function controller. Alternatively, an external device (4) can be connected to the data line (1), with the device being configured for the exchange of data with at least one of the controllers (2, 3, 3', 3") The data line is a line which is already in existence in the vehicle, particularly a K-diagnostics line and/or an L-diagnostics line in accordance with ISO standard 9141. For the control of the data exchange, one of the controllers (2, 3) connected via the data line (1) exercises the functions of the control unit.