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.
High severity. Can I keep driving?If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.
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
Most likely causes
- Starter/generator module unpowered
- CAN wiring to starter/generator broken
- Gateway module routing CAN messages incorrectly
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It affects emissions.
What each character of U0120 means
- U: Communication network
- 0: Generic code: same on every make
- 1: Subsystem depends on the code range
- 20: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0120?
If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.
What causes code U0120?
The most common causes are: starter/generator module unpowered; cAN wiring to starter/generator broken; gateway module routing CAN messages incorrectly.
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.