Code
U0251
Generic
U — Network/User
Lost Communication With Running Board Control Module B
Views:
UK: 16
EN: 29
RU: 18
AI status
Completed
Completed
100%
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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Code
U0251
HYUNDAI
U — Network/User
Lost Communication With Running Board Control Module “B”
Views:
UK: 8
EN: 17
RU: 12
AI status
Completed
Completed
100%
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
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email