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U0251 — Lost Communication Running Board Control Module B

Generic OBD-II · all makesNetwork / communication

U0251 indicates the vehicle's primary control module has lost serial data communication with Running Board Control Module B. This module operates the power-deployable running boards on one side of the vehicle, and the code sets when expected messages are missing on the CAN bus for a calibrated period.

Symptoms

What causes it — most likely first

1. Open or short in CAN bus wiring to Module B

Damaged, corroded, or disconnected CAN high/low circuits between the module and the gateway or splice pack are the most frequent root cause.

2. Running Board Control Module B internal failure

The module's transceiver or processor can fail due to moisture intrusion, vibration, or age, silencing its CAN transmissions.

3. Lost power or ground to Module B

A blown fuse, corroded power feed, or poor ground connection prevents the module from waking up and joining the bus.

4. CAN bus termination or splice pack fault

A faulty splice pack, missing termination resistor, or wiring harness damage elsewhere on the bus can isolate Module B.

5. Software mismatch or calibration issue

Incorrect module programming after a replacement or update can cause communication mismatches; reprogramming may resolve it.

6. Gateway or body control module hardware fault

Rare; a failed CAN controller in the gateway/BCM can prevent it from receiving Module B's messages.

How it's diagnosed

  1. Scan for additional U-codes; multiple lost-communication codes point to a bus-wide issue rather than a single module.
  2. Visually inspect the running board harness, connectors, and splice packs for damage, corrosion, or water intrusion.
  3. Verify battery voltage and ground at the Module B connector with a multimeter; check fuses in the associated circuit.
  4. Measure CAN bus resistance (approx. 60 ohms with ignition off) and check CAN high/low voltages (2.5 V recessive, 3.5 V/1.5 V dominant) at the module connector.
  5. Attempt bidirectional communication with Module B using a capable scan tool; if it responds, monitor live data for dropouts.
  6. If power, ground, and bus integrity are confirmed but the module remains silent, replace Running Board Control Module B and program per service information.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0249 — Lost Communication Entertainment ContrU0250 — Lost Communication Impact ClassificatiU0252 — Lost Communication Lighting Control MoU0253 — Lost Communication Accessory Protocol U0254 — Lost Communication Remote Start ModuleU0255 — Lost Communication Front Display Inter

Frequently asked

Will replacing the PCM/ECM fix U0251?

Almost never. U0251 is a local bus fault between the gateway/BCM and the running board module; the PCM/ECM is not on that communication path. Replace the PCM only if diagnostics prove its CAN controller is defective, which is extremely rare.

Can a dead battery or jump-start cause this code?

A low-voltage event can set temporary communication codes. Clear the code and retest after a full charge; if it returns, the fault is physical, not voltage-related.

Is it safe to drive with U0251 active?

The vehicle is drivable, but the affected running board will not deploy, creating a tripping hazard and possible door-ajar warning. Repair at your earliest convenience.

Do both running board modules need to be replaced together?

No. Modules A and B are independent; replace only the one that diagnostics confirm as faulty.