BU possesses diagnosing function which provides continuous control of entrance and output signals. In case of registration of failure which consequence excess of content of harmful substances in exhaust gases is BU includes the indicator lamp of failures signaling about malfunction existence. A number of emergency programs allows to drive the car and in case of failures in any of systems. Search of malfunctions is facilitated thanks to existence of the diagnostic socket to which the Volvo device for reading of codes of malfunctions
(tab. 9.1) is connected.