U01B5

Lost Communication With Auxiliary Body Control Module

U01B5 is a generic OBD-II network diagnostic trouble code: Lost Communication With Auxiliary Body Control Module. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U01B5
Group
Network
System
Network
Severity
Information only
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What U01B5 means

U01B5 is stored when a master control module -- often the main BCM or a gateway -- cannot receive expected messages from an Auxiliary Body Control Module. Auxiliary BCMs are supplemental nodes that expand body electrical capacity on feature-rich vehicles, typically handling zones or accessories not covered by the primary BCM, such as tailgate electronics, auxiliary lighting, heated seating zones, or running-board functions.

Because the affected module manages comfort and convenience features rather than safety-critical powertrain or braking functions, U01B5 is normally logged as an information or low-severity warning. The MIL may or may not illuminate depending on vehicle calibration; on some platforms only a body-system warning indicator is used. Symptom impact is confined to whatever functions the auxiliary BCM governs on that specific vehicle model.

The diagnostic path should start with a visual inspection of the auxiliary BCM's mounting location and connector, as these modules are often installed in areas prone to moisture (cargo floors, door sills, roof panels). Power supply, ground, and bus integrity checks follow. Because multiple modules may share the same CAN segment, a short on the bus can cause U01B5 alongside other U-codes simultaneously.

Common causes

Most-frequently reported root causes when U01B5 is logged.

  • 1
    Corroded or damaged harness connector at the auxiliary BCM.
  • 2
    Moisture intrusion into the auxiliary BCM or its connector.
  • 3
    Open or short in the data bus wiring to the auxiliary BCM.
  • 4
    Blown fuse or open ground for the auxiliary BCM supply circuit.
  • 5
    Failed auxiliary BCM hardware.
  • 6
    Bus fault on a shared network segment affecting multiple modules.
  • 7
    Auxiliary BCM not programmed after replacement.

Symptoms drivers notice

Body warning indicator or MIL illuminated.
Inoperative features managed by the auxiliary BCM (e.g., rear accessories, liftgate, or zone lighting).
No powertrain or safety-system symptoms.
Multiple U-codes may be present if a shared bus segment is faulted.

How to diagnose U01B5

A typical diagnostic flow when this code is present.

  1. 1
    Read and record all DTCs; multiple simultaneous U-codes suggest a shared bus or power fault.
  2. 2
    Identify the auxiliary BCM's location and function list from the vehicle service manual.
  3. 3
    Inspect the module and its connector for moisture, corrosion, or physical damage.
  4. 4
    Verify supply voltage and ground at the auxiliary BCM connector.
  5. 5
    Check bus termination resistance on the relevant network segment.
  6. 6
    Attempt scan tool communication with the auxiliary BCM to determine if it appears on the network.
  7. 7
    If wiring is sound, replace and program the auxiliary BCM per the manufacturer's procedure.

Related network codes

Frequently asked questions

Why does my vehicle have an auxiliary BCM in addition to the main BCM?

Complex vehicles with many electronic features require more input/output channels and processing capacity than a single BCM can provide. Manufacturers split body electrical functions across multiple modules to manage wiring complexity and allow scalable option packages.

Will U01B5 turn on the Check Engine light?

It depends on the manufacturer's calibration. Some vehicles illuminate the MIL for any U-code; others only activate a body warning indicator. Either lamp will cause an OBD-II emissions test failure.

Can a water-damaged auxiliary BCM be dried and reused?

In most cases no. Once moisture enters a BCM and causes internal corrosion, reliability cannot be guaranteed. Replacement is the standard repair.

Is U01B5 related to U01B0 or other nearby codes?

Nearby codes in the U01Bx range identify different specific modules. U01B5 is specific to the Auxiliary Body Control Module. Always use the full five-character code when cross-referencing.

Disabling U01B5 in software

RaceTune can permanently disable U01B5 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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