U0251: Lost Communication With Running Board Control Module B

What it means

A control module has stopped receiving expected messages from running board control module B. The code identifies the communication loss; powered running-board functions may be affected if equipped.

How it works

Running board control module B communicates with other modules to support powered running-board operation when the vehicle is equipped with that system. U0251 sets when expected messages from module B are no longer received, which may make its assigned running-board functions unavailable.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Powered running-board functions may be unavailable or limited if equipped
• A warning may appear depending on the vehicle

How to diagnose it

1. Scan all modules and save freeze-frame and communication data to see which module logged U0251 and whether other network faults are present.
2. If equipped, check whether the running-board functions associated with module B respond, keeping clear of moving boards.
3. Inspect module B's connector and nearby wiring for impact damage, water, corrosion, loose plugs, or harness chafing.
4. Check module B's power and ground according to the manufacturer's procedure.
5. Test the network wiring to module B for opens or shorts using the correct wiring diagram and test method.
6. If supplies and wiring are sound, perform the manufacturer's communication checks before replacing module B.

Common fixes

• Secure a loose connector at module B.
• Repair damaged or corroded network wiring and connector terminals.
• Restore a failed power feed or ground to module B.
• Replace module B only after its power, ground, and network connections test correctly.

Can I keep driving?

The fault may disable powered running-board functions if fitted. Keep clear while checking operation, and do not rely on a board to support entry or exit until you know it is stable.