Lost Communication With Starter / Generator Control Module
The U0120 code means the vehicle's onboard diagnostic system has detected: Lost Communication With Starter / Generator Control Module. This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- $50 - $600 (fuses, grounds, or wiring most often; module replacement only after the circuit tests good)
Network codes mean modules are not communicating. You need a scanner that reads all systems to find the problem.
Affiliate links — we earn a commission at no extra cost to you.
Common Symptoms
- Check Engine or related warning light on
- The Starter / Generator Control Module's functions may be degraded or offline
- Often stored alongside other U-codes when a whole bus segment drops
- Symptoms can be intermittent - connector and wiring faults come and go with heat and vibration
Probable Causes (Ranked by Likelihood)
- The Starter / Generator Control Module has lost power or ground (check its fuses first) High
- CAN bus wiring damage - chafed harness, corroded connector, water intrusion High
- Failed Starter / Generator Control Module no longer transmitting Moderate
- Another faulted module loading down the bus Low
Frequently Asked Questions
What does U0120 mean?
U0120 means other modules stopped receiving messages from the Starter / Generator Control Module on the vehicle network. It is a communication fault - the module may be fine but unreachable.
Can I keep driving with U0120?
Usually yes for a short period. The affected function may be degraded, and intermittent wiring faults tend to worsen - diagnose soon.
Should I replace the module for U0120?
Not first. Most communication codes are power, ground, fuse, or wiring problems. Replace the module only after those test good - modules are expensive and often require programming.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
-
Scan ALL modules - the pattern of which modules report lost communication localizes the break.
-
Check the fuses and grounds feeding the Starter / Generator Control Module before suspecting the module itself.
-
If many modules are unreachable, test the CAN bus: about 60 ohms across CAN High/Low (two 120-ohm terminators) with the battery disconnected.
-
Wiggle-test connectors along the affected harness while watching for the module to drop on and off.
-
Only after power, ground, and bus test good should the Starter / Generator Control Module be suspected.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
Commonly Affected Vehicles
- All OBD-II vehicles - network (U) codes are standardized across makes
Related Codes
Explore More
Was this helpful?
Send this to whoever is fixing the car — your mechanic, or the person asking what the light means.