What it means
A control module received invalid data from the brake blending actuator. The code identifies a data validity fault; the specific affected function depends on the vehicle.
At a glance
- Usually lights the warning light of the system that lost communication
- The engine or gearbox may go into limp mode
How it works
The brake blending actuator helps coordinate friction braking with regenerative braking where the vehicle has that function. U04B4 is set when a receiving module considers data from the actuator invalid, so braking or energy recovery may be limited even though the code alone does not prove the actuator has failed.
Common causes
Most likely first.
- Fault in the transmitting module
- Network wiring or connector fault
- Module power or ground fault
- Software or configuration mismatch
Symptoms
- A brake-system warning may appear
- Braking or regenerative braking functions may be affected
How to diagnose it
- Record freeze-frame data and scan all brake, stability, and hybrid modules for related codes before clearing the fault.
- Check live data for brake blending actuator status and identify which reported values are absent, implausible, or flagged invalid.
- Inspect the actuator connector and nearby network wiring for damage, corrosion, loose pins, or signs of water intrusion.
- Check the actuator's power and ground against the manufacturer's specification.
- Test the network wiring between the actuator and receiving modules for poor connections, opens, or shorts using the vehicle wiring information.
- If wiring and supplies are sound, check applicable software or configuration requirements before replacing the actuator or a module.
Common fixes
- Reconnect or repair a loose or corroded brake blending actuator connector.
- Repair damaged network wiring or connector terminals.
- Restore the actuator's faulty power or ground connection.
- Correct an applicable software or configuration issue.
- Replace the actuator or transmitting module only after diagnosis confirms the source of invalid data.
Can I keep driving?
Do not drive if braking feels different, a brake warning is on, or regenerative braking changes unexpectedly; have the vehicle recovered if normal braking is uncertain. A fault in brake blending can affect braking functions, so arrange prompt diagnosis even if the vehicle still stops.
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.