OBD-II · Network and communication (U)
U0001
High-speed CAN communication bus fault
What it means
A control module has detected a problem on the high-speed CAN data network that the car's main computers use to talk to each other.
Symptoms
- Several warning lights on at once
- Gauges or displays dropping out
- Car may not start or may go into limp mode
- Other U codes stored in several modules
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | weak or failing 12V battery, or a recent flat battery / jump start |
| 02 | corroded or loose connector on the CAN wiring (water ingress under seats, in footwells or at the gateway) |
| 03 | chafed or shorted CAN wire |
| 04 | a faulty module pulling the whole bus down |
| 05 | poorly installed aftermarket accessory (tracker, alarm, dashcam, radio) spliced into CAN |
What to check, step by step
- 01Record all module codes and missing modules before clearing anything; test the 12V battery and charging system
- 02Inspect the affected network for water ingress, damaged wiring and aftermarket connections
- 03Use the OEM network diagram to identify the affected bus and its test points; the diagnostic socket may be separated from it by a gateway
- 04Measure resistance only on a de-energised circuit following OEM instructions; about 60 ohms applies to a bus with two 120-ohm terminators, not every network
- 05Isolate branches or modules only using the OEM power-down and disconnection procedure, accounting for termination resistors; observe SRS and high-voltage precautions where applicable
- 06After repair, clear recorded codes and rescan all modules