There is disclosed a self-arming immobilizer system for immobilizing a vehicle. In an embodiment, the system comprises a signal input for receiving a signal indicating the absence of an authorized operator in a position of control in the vehicle; and an immobilizer unit adapted to receive the signal input, and in response to the signal indicating the absence of an authorized operator in a position of control in the vehicle, switch one or more relays to disable one or more circuits in the vehicle to immobilize and secure the vehicle. In an embodiment, the immobilizer unit is adapted to disable a gear shift link solenoid to disable the gear shift. In another embodiment, the immobilizer unit is adapted to disable a gun rack solenoid to lock the gun rack. In another embodiment, the vehicle is immobilized at idle, and the system allows the vehicle to keep idling until deactivated by an authorized operator.