U01B4

Code

Generic U — Network/User

Lost Communication With Brake System Control Module B

Brand: Generic

Type: U — Network/User

Views: UK: 29 EN: 55 RU: 30

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in the CAN bus wiring to Brake System Control Module B
Poor or corroded connector or pin at the brake module
Module powered down due to blown fuse, low battery, or supply problem
Missing or incorrect bus termination (open/incorrect resistor)
Software/configuration mismatch or failed module programming
Internal module failure

Symptoms

ABS/ESC warning lamp illuminated, brake system warning lamp
Loss of some brake-related functions (ABS/traction control may be disabled)
Related systems (stability control, cruise) may be disabled or show faults
Cannot communicate with Brake System Control Module B with a scan tool
Multiple network or U-codes stored on different modules

What to check

Record all stored codes from all modules and note freeze frame data
Verify battery voltage is within specification and charging system is normal
Inspect fuses and power/ground circuits for the brake module
Visually inspect harness and connector at the brake module for damage or corrosion
Check for other network-related codes (U0xxx) that indicate wider bus issues
Measure CAN-H and CAN-L voltages at the brake module connector with ignition ON

Signal parameters

High-speed CAN idle: CAN‑H ≈ 2.5 V, CAN‑L ≈ 2.5 V (recessive). During dominant bits CAN‑H ≈ 3.5 V and CAN‑L ≈ 1.5 V (differential ≈ 2.0 V)
Typical termination: two 120 ohm resistors in parallel (≈60 ohms) across CAN‑H and CAN‑L
ABS/brake module should transmit periodic status frames; absent frames or continuous error frames indicate comms fault
Watch for excessive bus error frames, bus-off conditions, or nodes reporting high error counters

Diagnostic algorithm

Record all current and historic codes from all control modules; note when code set and any related symptoms.
Verify battery and charging voltages; clear codes and attempt to reproduce. Some comms faults require correct supply voltage.
Attempt to communicate with Brake System Control Module B using a capable scan tool. Note if module is detected or not.
Inspect module connector: disconnect, inspect pins for corrosion, damage or pushed-out pins, and check mating harness.
Check fuses and measure module supply and ground circuits for proper voltage and good resistance to chassis ground.
With ignition ON, measure CAN‑H and CAN‑L at the module connector. Compare to expected idle voltages and to another known-good node.
Measure resistance between CAN‑H and CAN‑L with power off to confirm termination ≈60 ohms. If open/shorted, trace harness.
If voltages are abnormal, isolate by disconnecting nearby modules one at a time while monitoring bus behavior to locate a shorted node.
Use a scope to view CAN waveforms if available; look for noise, missing frames, or dominant states that indicate a short or stuck node.
Repair any wiring, connector, power/ground or termination faults found. After repair, clear codes and verify communication is restored and no other U-codes remain.
If wiring and power/ground are good and problem persists, check for available software updates or reprogramming procedures for the brake module per manufacturer guidance before replacing the module.
If replacement is required, program/configure the new module as specified and confirm proper network operation.