U01B4Lost Communication With Brake System Control Module B
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- MIL on, sometimes with brake or stability warnings
- Driver assistance functions unavailable
- Changed brake pedal feel if the booster is affected
- Likely causes
- 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
- RareBrake system control module B faulty
- Repair
- Medium · Usually a job for a workshop.Typical cost: €0 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.