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DTC codes / U01xx · Lost communication with a control module / U01B4

U01B4Lost Communication With Brake System Control Module B

U01B4 is a generic network communication code indicating that the primary control module (typically the engine control module or a gateway module) has lost contact with the brake system control module B over the vehicle's controller area network (CAN) bus. The computer has detected a lack of expected periodic messages from that specific brake module, which may be due to a wiring fault, a module failure, or a power/ground issue.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaLost communication with a control module

Symptoms you may notice

  • Illumination of the anti-lock brake system (ABS) warning light, traction control light, and/or brake system warning light on the dashboard.
  • Often no symptoms besides the warning light, with normal brake pedal feel and braking performance.
  • In some cases, reduced or disabled ABS and traction control functions, especially during slippery conditions.
  • Intermittent or permanent loss of communication with a scan tool when attempting to access the brake system control module.
  • Possible activation of a limp-home mode or reduced power if the brake module shares critical data with the engine or transmission.
  • Other unrelated warning lights may appear if the brake module is part of a network segment that also hosts other modules.

Common causes, most likely first

  1. Damaged, corroded, or disconnected wiring or connectors in the CAN bus circuit between the brake system control module B and the rest of the network.
  2. Faulty brake system control module B itself, due to internal electronic failure or water intrusion.
  3. Blown fuse or faulty relay supplying power to the brake system control module B.
  4. Poor ground connection for the brake system control module B, leading to intermittent or total loss of communication.
  5. CAN bus network faults such as a short to ground or voltage, or a broken wire, affecting the entire bus or a segment.
  6. Software or configuration mismatch in the brake module after a replacement or reprogramming event.
  7. Another module on the same CAN segment causing bus disruption (e.g., a faulty module transmitting garbage or holding the bus low).

How to diagnose and fix it

  1. Verify the code with a scan tool that can access all modules, and check for communication with the brake system control module B to confirm the fault is current.
  2. Inspect the battery, charging system, and all fuses and relays related to the brake system control module B; replace any blown fuses or faulty relays.
  3. Visually inspect the wiring harness and connectors at the brake module for corrosion, chafing, or loose pins; repair or replace as needed.
  4. Perform a voltage drop test on the power and ground circuits to the brake module to rule out high resistance connections.
  5. Use an oscilloscope or a CAN bus analyzer to check for proper differential voltage signals on the CAN bus; look for shorts, opens, or excessive noise.
  6. If the bus and wiring are intact, substitute a known-good brake system control module B (if possible) or perform a module swap with proper reprogramming to confirm the fault.
  7. After repair, clear the code and perform a road test to verify that communication is restored and no other codes return.

Common Misdiagnosis Traps for U01B4

  • A weak or failing battery can cause intermittent communication loss on the CAN bus, leading to U01B4 without any actual module or wiring fault.
  • The code may set due to a faulty ground connection at the brake module, which is often overlooked because the ground wire appears secure but has high resistance.
  • After replacing a brake system control module, failing to program or configure it correctly can cause U01B4 to reappear immediately.
  • U01B4 can be a secondary code triggered by a fault in another module on the same CAN segment, so always check for other communication codes before condemning the brake module.
  • Some vehicles use a dedicated brake module that is part of the stability control system; a failing wheel speed sensor or steering angle sensor can sometimes cause the module to shut down communication, setting U01B4.

Can I keep driving with U01B4?

If the brake warning light is on and ABS/traction control is disabled, drive with extra caution, especially in wet or slippery conditions, and avoid hard braking. It is generally safe to drive to a repair shop, but if the brake pedal feels abnormal or other critical warnings appear, stop driving and seek professional assistance immediately.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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