What it means
Vehicle communication bus B performance is outside the monitored range. The code does not identify a specific cause.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
Bus B is the vehicle network linking its connected control modules. U0038 indicates that monitored communication performance on this bus is outside the expected range, such as messages or signal behavior becoming unreliable; it does not point to one component by itself.
Common causes
Most likely first.
- Wiring chafed or marginal
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Record all codes and freeze-frame data, noting when U0038 occurs and which bus B modules report related communication faults.
- Check scan-tool live data or network status for intermittent message errors and identify whether they occur across bus B or around one module.
- Inspect bus B wiring along its accessible route for chafing, pinched sections, loose connectors, corrosion, or water intrusion.
- Check system voltage and module power and grounds, since unstable supply can make bus communication unreliable.
- Test bus B wiring, termination, and gateway routing against the manufacturer's specifications, then isolate a suspect node only using the service procedure.
Common fixes
- Secure or repair chafed bus B wiring and loose connectors.
- Clean or repair corroded connections and restore affected module power or grounds.
- Replace a terminating resistor only if its measured value is outside the manufacturer's specification.
- Repair gateway routing or replace a bus-connected module only when testing confirms the fault.
Can I keep driving?
If the fault coincides with loss of steering, braking, stability control, or another essential system, stop driving and arrange recovery. If no essential function is affected, drive cautiously and have the intermittent communication fault diagnosed soon because it can disable connected 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.