Abstract: A control system comprises a controller for controlling an actuator, for instance of an electrically power assisted vehicle steering system. An unstable circuit, such as first and second cascade connected integrators has an input connected to the controller and an output connected to a detector, which signals an error when the output signal of the unstable circuit is outside an acceptable range. During normal operation, the controller supplies a signal to the unstable circuit such that its output signal remains within the acceptable range. The error signal may be used to disable the actuator, for instance so as to remove power assistance from steering system.