U0DB5
Lost Communication With Electric Motor Control Module BU0DB5 is a generic OBD-II network diagnostic trouble code: Lost Communication With Electric Motor Control Module B. 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.
What U0DB5 means
U0DB5 is stored when the vehicle control network loses communication with electric motor control module B, the second of two independent motor control ECUs on vehicles equipped with dual electric motor drivetrains, such as all-wheel-drive battery electric vehicles or certain hybrid architectures. Module B typically governs the rear or secondary motor and reports torque output, speed, and fault status to the propulsion management system.
With module B offline, the vehicle management system cannot confirm or command the secondary motor's torque contribution. Most platforms respond by disabling the affected motor and operating in front-wheel or single-motor mode, which means significantly reduced performance, loss of AWD capability, and potential traction stability limitations. In some configurations the vehicle may be placed in a full limp mode if the system cannot safely balance torque without the secondary motor's feedback.
Diagnosis follows the same high-voltage safety framework as for U0DA8 (motor control module A). The 12V control supply to module B, its CAN bus wiring, and the high-voltage interlock loop should all be verified before condemning the module. The geographic separation of module B from module A means wiring faults specific to the rear or secondary drivetrain area should be investigated, including any damage from impacts, water ingress, or heat.
Common causes
Most-frequently reported root causes when U0DB5 is logged.
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1
Loss of 12V control power or ground to electric motor control module B.
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2
Open or shorted CAN bus wiring on the module B network branch, often at the rear axle.
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3
Failed electric motor control module B no longer transmitting on the vehicle network.
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4
Overtemperature shutdown of module B due to sustained high load on the secondary motor.
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5
High-voltage interlock loop (HVIL) fault at a rear high-voltage connector.
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6
Corroded or damaged connector at module B, particularly in rear undercarriage locations.
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7
Impact damage or water ingress into the secondary motor assembly or its control module.
Symptoms drivers notice
How to diagnose U0DB5
A typical diagnostic flow when this code is present.
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1
Apply full high-voltage safety precautions per the manufacturer service procedure before beginning.
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2
Use a manufacturer-specific or enhanced scan tool to retrieve all DTCs; document all co-set network and drivetrain codes.
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3
Verify 12V supply voltage and ground at the electric motor control module B connector.
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4
Inspect the module B connector and harness at the rear axle area for corrosion, water damage, or impact damage.
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5
Measure CAN bus differential voltage at the module B connector with the ignition on.
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6
Check the high-voltage interlock loop continuity at all rear high-voltage connectors if applicable.
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7
If power, ground, and CAN signal are confirmed but module B does not respond, replace the module per high-voltage service procedures.
Related network codes
- U0D88 — Lost Communication With Battery Energy Control Module 'D'
- U0D8B — Lost Communication With Battery Energy Control Module 'G'
- U0D8C — Lost Communication With Battery Energy Control Module 'H'
- U0D8D — Lost Communication With Battery Energy Control Module 'I'
- U0D8E — Lost Communication With Suspension Control Module
- U0D90 — Lost Communication With Trailer Brake Control Module
Frequently asked questions
What is the difference between U0DA8 and U0DB5?
U0DA8 refers to the primary or first electric motor control module (module A), while U0DB5 refers to the secondary motor control module (module B). On dual-motor AWD vehicles both modules are present; each has its own network node and communication code.
Can the vehicle still drive with U0DB5 active?
On most dual-motor platforms the vehicle can continue in front-drive or single-motor mode, but AWD and full power are unavailable. Some platforms apply a more restrictive limp mode. Check the instrument cluster message for guidance.
Is the rear motor mechanically disabled when U0DB5 is active?
Not necessarily mechanically, but the high-voltage drive to the motor is typically cut by the inverter or motor control system as a safety measure when the control module is not communicating. The motor will rotate passively with the wheels but will not generate torque.
Can moisture in the rear module cause intermittent U0DB5?
Yes. The rear motor control module is in an exposed undercarriage location on many AWD electric vehicles and is susceptible to water ingress, especially after pressure washing or deep water crossings. Inspect the module and connector for moisture if the code appears intermittently.
Disabling U0DB5 in software
RaceTune can permanently disable U0DB5 — 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.
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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