Lost Communication With Starter / Generator Control Module
Your vehicle's main computer lost its connection to the starter/generator control module, like a phone losing signal to a tower. This communication breakdown prevents proper charging and starting system management.
The ECM monitors CAN bus communication frames from the starter/generator control module at regular intervals. When expected messages fail to arrive within the timeout window, the ECM flags a communication loss. This typically occurs when the module stops responding to bus queries or when signal integrity degrades.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| CAN Bus Message Timeout | Response within 100-500ms per module | No response after 2-3 consecutive cycles |
| Signal Integrity | CAN voltage 0-5V with clean transitions | Noise, dropout, or bus voltage collapse |
Code U0120 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, U0120 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.