What it means
A fault is detected on the negative conductor of vehicle communication bus H; the code descriptor specifies a high condition. Which functions are affected depends on the vehicle’s network design.
How it works
Modules on bus H exchange data through paired positive and negative conductors. U0099 means the negative conductor is detected at an unexpectedly high level, a condition that can prevent modules on this network from exchanging messages correctly.
Common causes
Most likely first.
- Short to voltage or damaged bus H negative wire
- Loose or corroded connector
- A module or gateway fault
Symptoms
- Communication faults in modules using bus H
- Warning lights or messages
- Functions relying on bus H may be unavailable
How to diagnose it
- Scan all modules for U0099 and related communication faults, then note which bus H modules are affected and when the fault was recorded.
- Locate the bus H negative wiring and inspect it and its connectors for contact with a voltage source, damaged insulation, corrosion, or displaced pins.
- Use the manufacturer's procedure to monitor or measure the negative conductor and verify that it is higher than expected.
- Check the negative wire for a short to voltage and for continuity, isolating the circuit only as directed in the service information.
- If the conductor and connectors pass inspection and testing, check the gateway and connected modules for a fault driving the negative line high.
Common fixes
- Secure or clean a loose or corroded bus H connector.
- Repair the negative conductor where it is damaged or shorted to voltage.
- Repair or replace the gateway or module confirmed to be causing the high condition.
Can I keep driving?
If the vehicle operates normally, restrict driving to essential trips until the communication fault is diagnosed. Stop if a safety-related function becomes unreliable, as modules sharing bus H may lose their messages.
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.