U0120: Lost Communication With Starter / Generator Control Module

U0120 is a network communication code that records when the vehicle's communication bus has lost contact with the starter or generator control module. Modern vehicles rely on a shared data bus (typically CAN bus) to allow modules to talk to each other, and the starter/generator control module is one of the nodes on that network. When other modules stop receiving expected messages from it, U0120 gets logged.

The most common triggers are a blown fuse supplying the starter/generator control module, a loose or corroded connector on that module, damaged wiring in the CAN bus harness near that node, or a failed module itself. Because this is a communication fault, it often appears alongside other U-codes if the issue is with the bus itself rather than the individual module. Checking the module's power and ground supply first is the right approach, followed by inspecting the CAN bus wiring for any obvious damage.

U0120 is rated high severity because the starter and generator controls sit in the powertrain. The module itself may still be working and only the communication interrupted, but you cannot tell which from the code alone. That said, a loss of communication with charging or starting system controls can affect how the vehicle manages electrical load, so it is worth having it investigated. Do not assume it will resolve itself without checking the physical connections.