U01B4 code meaning
Lost Communication With Brake System Control Module B
Network system Generic code U01xx codes Check engine light
What U01B4 means
Brake system control module B is the second brake controller in a two-unit layout, often the electric brake booster beside the stability control unit. GM's 2018 table for the CT6 plug-in hybrid (18OBDG01) has the engine controller flag U01B4 when frame $4FB from BSC2 has been missing for 10 s on the high-speed bus, Type B, two trips. Porsche technical information 105/25 (Taycan from 2020, Cayenne and Panamera from 2024) shows U01B400 in the PSM unit as lost contact with the brake booster, paired with C114113 when the booster's roughly one-minute run-on after switch-off was cut short; the MIL lights, yet no part needs replacing, erasing the memory with a charger connected suffices. U0129 is module A, U04B5 invalid data from B.
U01B4 at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.3 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of U01B4
- MIL on, sometimes with brake or stability warnings
- Driver assistance functions unavailable
- Changed brake pedal feel if the booster is affected
Common causes of U01B4
- Most likelyBooster run-on interrupted at switch-off (Porsche, no part fault)
- PossibleDamaged CAN wiring or connector at the brake module
- PossibleLow 12 V supply during start or shutdown
- Less likelyBrake system control module B faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake system module bCAN bus
What each part of U01B4 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 U01B4 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 U01B4
What does U01B4 mean?
U01B4 means "Lost Communication With Brake System Control Module B". Brake system control module B is the second brake controller in a two-unit layout, often the electric brake booster beside the stability control unit. It is a generic code, so it means the same on every make.
Is it safe to drive with U01B4?
Drive gently and only to a workshop. With U01B4 the car can go into limp mode (reduced power) to protect itself.
Will U01B4 fail an emissions test?
U01B4 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U01B4 to fix?
Difficulty: medium. A typical repair takes 0.3 to 3 hours of work. This is a job for a workshop.
Will U01B4 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.