CAN Data Link
WARNING: This page is about a different car, the 2009 Audi A4, 2008 Audi A4, 2007 Audi A4, 2006 Audi A4, and 2005 Audi A4. However, it is still accessible from the selected car via links, so may be relevant.
CAN is an acronym for Controller Area Network.
The ECM communicates with all databus capable control modules by a CAN Data Link.
The databus capable control modules (i.e. Engine Coolant temperature Sensor) are connected by two data bus wires which are twisted together (CAN_High and CAN_Low), and exchange information (messages) to the ECM. Missing information on the databus is then recognized and stored as a malfunction.
The ECM illuminates the MIL through the CAN data link and tells the MIL to turn on, turn off, or blink.