What it means
A fault is detected on the negative conductor of vehicle communication bus G; the code descriptor specifies a high condition. Which functions are affected depends on the vehicle’s network design.
How it works
Bus G uses positive and negative conductors to carry messages between its connected control modules. U0090 is set when the negative conductor is detected at a higher level than expected, potentially because it is contacting voltage and disrupting network communication.
Common causes
Most likely first.
- Short to voltage or damaged bus G negative wire
- Loose or corroded connector
- A module or gateway fault
Symptoms
- Communication faults in modules using bus G
- Warning lights or messages
- Functions relying on bus G may be unavailable
How to diagnose it
- Scan all modules for bus G communication codes and identify which units are missing or reporting network faults.
- Check event data and live network status for whether module dropouts match the negative-line high condition.
- With power isolated as specified by the manufacturer, inspect bus G negative wiring and connectors for contact with powered wiring, chafing, corrosion, or loose pins.
- Test the negative conductor for a short to voltage and verify continuity through suspect connectors and wiring using the manufacturer's procedure.
- If the harness checks out, follow the manufacturer's module isolation procedure to find a module or gateway causing the negative conductor to read high.
Common fixes
- Clean and secure a loose or corroded bus G connector.
- Repair chafed bus G negative wiring that contacts a voltage source.
- Replace damaged terminals or connectors on the bus G negative circuit.
- Repair or replace a module or gateway confirmed to be driving the negative line high.
Can I keep driving?
Bus G dependent functions may be lost or warning messages may appear; whether driving is safe depends on which systems use that network. Avoid driving if safety-critical functions are affected or their status cannot be confirmed.
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.