Network, generic

U01B4 Lost Communication With Brake System Control Module B

Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Booster run-on interrupted at switch-off (Porsche, no part fault)
Damaged CAN wiring or connector at the brake module
Low 12 V supply during start or shutdown
Brake system control module B faulty

Symptoms

  • MIL on, sometimes with brake or stability warnings
  • Driver assistance functions unavailable
  • Changed brake pedal feel if the booster is affected

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–1,500
Time0.3–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake system module bcan bus

Related codes

U0129U04B5U031CU015AU025E

References: gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov, bosch-mobility.com.