No Start No Crank Intermittently / No Communication Over High Speed LAN - keywords connector inop inoperative lamp light loose message on OnStar VCIM warning U0121 U0101 U2100 U1040 P0700 P2544 U0073

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

A customer may have a complaint of an intermittent no start, no crank condition, and or SES light on with I/P gauges intermittently inoperative while driving. Any of the following DTCs may be accompanied with this complaint. U0121, U0101, U2100, U104, P0700, P2544, U0073

Before beginning to deep dive into any diagnostics first check the VCIM (Onstar) connector C3 (Black) to be sure that it has been fully seated. This connector has been increasingly showing up as the root cause for loss of HIGH speed LAN concerns.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.