U04B4 code meaning
Invalid Data Received From Brake Blending Actuator
Network system Generic code U04xx codes Check engine light
What U04B4 means
In hybrid and electric cars, brake blending decides how much of the requested deceleration the drive motor recovers as energy and how much the friction brakes supply; at low speed the hydraulic brakes take over completely. An electromechanical booster such as Bosch's iBooster, paired with a matching ESP, is rated for blending up to about 0.3 g of regenerative deceleration. U04B4 is stored when messages from the blending actuator arrive but their values are not credible. No manufacturer documents this code publicly. CARB 1968.2 requires hybrids to flag faults that disable or degrade regenerative braking, so a MIL is possible. U01B3 marks a silent actuator. Expect less regeneration, a changed pedal feel and a brake warning.
U04B4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U04B4
- Brake system or hybrid warning light on
- Little or no regenerative braking, shorter range
- Brake pedal feels different or harder
Common causes of U04B4
- PossibleBlending actuator or its pedal/position sensing faulty
- PossibleChassis bus wiring or connector fault
- PossibleBrake and drive controller software not matching after update or replacement
- Less likelyLow 12 V supply affecting the actuator electronics
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake blending actuator
What each part of U04B4 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| B4 | The exact fault in that area. |
What to do next
- Note U04B4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U04B4
What does U04B4 mean?
U04B4 means "Invalid Data Received From Brake Blending Actuator". In hybrid and electric cars, brake blending decides how much of the requested deceleration the drive motor recovers as energy and how much the friction brakes supply; at low speed the hydraulic brakes take over completely. It is a generic code, so it means the same on every make.
Is it safe to drive with U04B4?
Drive gently and only to a workshop. With U04B4 the car can go into limp mode (reduced power) to protect itself.
Will U04B4 fail an emissions test?
Yes, it can. U04B4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U04B4 to fix?
Difficulty: workshop only. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U04B4 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.