Abstract: A vehicle alarm comprises of a control unit (2) coupled to a message center (22) and an ultrasonic movement sensor (20). The ultrasonic movement sensor (20) is automatically activated when the vehicle ignition is switched off. If movement is not detected in the time period between switching the ignition off and the driver's door being closed, then the ultrasonic movement sensor (20) is deemed to have failed. A count of such failures is kept and when a predetermined number of such failures has occurred, a warning message is displayed on the message center (22). If movement is detected in the time period, the count is reset to zero. In this way, the alarm can automatically determine whether or not the ultrasonic movement sensor (20) is functioning correctly.