CAN bus
Controller Area Network — inter-ECU bus.
CAN is the serial bus that ECUs use to talk to each other (mandatory since 2008 in the US). Speeds: 500 kbps (HS-CAN) or 125 kbps (MS-CAN). Loss of CAN communication with an ECU yields a U0xxx code (e.g., U0100 = "lost communication with ECM").