U0120
Lost Communication with Starter/Generator Control Module
The Starter/Generator Control Module has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. Stop-start and mild-hybrid generator functions stop working. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- Starter/generator module unpoweredCommon
- CAN wiring to starter/generator brokenPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- Starter Generator Module
- CAN Bus Wiring
U0120 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.