U0251

Code

Generic U — Network/User

Lost Communication With Running Board Control Module B

Brand: Generic

Type: U — Network/User

Views: UK: 25 EN: 42 RU: 29

AI status

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Page language: EN

Causes

Failed Running Board Control Module B (internal fault)
Open or short in CAN/LIN data wiring to the module
Poor or missing power or ground to the module
Corroded or damaged connector pins or sockets
Blown fuse or power distribution fault for the module
Bus short to battery or ground, or missing bus termination

Symptoms

Running board functions (deploy/retract) inoperative or erratic
Instrument cluster/scan tool shows U0251 or a running board communication warning
Related vehicle features tied to the module do not operate
Possible multiple network errors or other modules reporting communication faults
No response when attempting to communicate with the module via scan tool

What to check

Retrieve freeze frame and stored U-codes; check for related codes in other modules
Attempt to communicate with RBM B using a compatible scan tool
Visually inspect RBM B connector and wiring for corrosion, damage, or moisture
Check fuses and power distribution for the running board module circuit
Verify module key-on and constant power, and verify ground integrity at the module connector
Backprobe CAN (or LIN) data pins at the module and compare to expected voltages

Signal parameters

If CAN network: CANH and CANL idle ~2.5 V each; dominant state typically shifts CANH higher and CANL lower (differential ~1–3 V)
Common CAN data rates: 125 kbps, 250 kbps, 500 kbps or 1 Mbps — verify vehicle-specific rate
Bus termination: two 120 ohm resistors in parallel ≈ 60 ohms between CANH and CANL
LIN (if used): single-wire serial bus ~0–12 V levels depending on vehicle — check vehicle service info
Look for missing or noisy signals, short to battery or ground, or no toggling on the bus when commands are sent

Diagnostic algorithm

Connect a professional scan tool and confirm U0251 and any related codes. Note vehicle history and freeze frame data.
Attempt to communicate directly with the Running Board Control Module B. If communicable intermittently, record conditions (key state, vehicle movement, door status).
Inspect module connector and wiring for corrosion, bent pins, water intrusion, or damage. Repair any visible faults.
With ignition off, measure resistance between CANH and CANL at module connector (~60 ohms). If open or shorted, trace wiring to locate break or short.
With ignition on, backprobe CANH and CANL (and power/ground) at the module connector. Verify proper key-on voltage, ground, and that the bus shows expected voltages and activity with a scope or frequency tool.
If bus signals are absent at the module but present at other nodes, perform continuity checks between the module connector and the network junctions. Repair wiring as needed.
If bus shows contention or abnormal voltages, isolate by disconnecting other nodes one at a time to find the offending device or short.
If power/ground missing, locate and check fuses, relays, and related wiring; restore proper power/ground.
If wiring and power/ground are good but module does not respond, confirm correct programming/configuration for the vehicle and consider replacing the module per manufacturer procedure.
After repair or replacement, clear codes and verify normal operation and that no new communication codes return.

Likely causes

Connector corrosion or bent pins at the running board module
Open/short in CAN high/low wiring between module and bus
Module has no key-on power or missing ground
Bus terminated incorrectly (open or shorted termination resistor)
Intermittent harness/connector causing intermittent communication loss

Fault status

⚠️ Status

Lost Communication With Running Board Control Module B — module not responding on vehicle data bus (CAN/LIN).

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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