A 2003 Saturn L300 came in with CAN BUS communication problems producing codes U1000 and a U2105 (Figure 1). U1000 is a Class II Data Link code. One explanation given for this code by GM is that should this code be set in history with other DTCs set in “current or history,” to diagnose these codes or code first. This means DTC U2105 could be the cause of code U1000 and will need to be diagnosed and resolved first. It is learned that code U2105 will set if the BCM or TCM lost CAN communications with the ECM.