What it means
A fault is detected between the positive and negative conductors of vehicle communication bus G. Which functions are affected depends on the vehicle’s network design.
How it works
Bus G carries messages over a paired positive and negative conductor between connected modules. U0091 indicates those two conductors are shorted together, which can corrupt messages or prevent modules on the network from communicating.
Common causes
Most likely first.
- Bus wires shorted together
- Damaged or chafed wiring
- Connector damage or water intrusion
- Faulty module connected to bus G
- Incorrect wiring repair
Symptoms
- Communication warning lights
- Multiple warning messages
- Intermittent system operation
- Some systems unavailable
How to diagnose it
- Scan all modules for bus G faults and identify which units are offline or intermittently reachable to locate the affected network branch.
- Review event data and live communication status for when the conductor-to-conductor fault appears and whether it follows vibration or moisture.
- With power isolated as the manufacturer directs, inspect bus G's paired wires and connectors for chafing, pinched sections, water intrusion, or incorrect repairs.
- Test for an unintended connection between bus G positive and negative and check each conductor for continuity using the manufacturer's procedure.
- If the harness is clear, isolate connected modules and the gateway one at a time as directed to determine whether a faulty unit is shorting the pair.
Common fixes
- Dry and clean a connector with water intrusion, then repair any damaged seals or terminals.
- Repair chafed or pinched bus G wiring that lets the paired conductors touch.
- Correct an improper wiring splice or replace a damaged connector.
- Repair or replace a module confirmed to be shorting bus G positive to negative.
Can I keep driving?
Intermittent or lost communication can disable functions on bus G, and the affected systems may change as the fault comes and goes. Avoid driving if the fault affects essential controls or safety systems.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.