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Home / DTC / U0251 — Lost Communication With Running Board Control Module “B”

U0251 — Lost Communication With Running Board Control Module “B”

Detailed page for trouble code U0251.

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Code

U0251

HYUNDAI U — Network/User

Lost Communication With Running Board Control Module “B”

Brand: HYUNDAI
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Blown fuse or relay supplying running board control module
  • Module ground or battery feed open/high resistance
  • Loose, corroded or damaged connector at running board control module
  • Broken, shorted or high-resistance CAN wiring (CAN_H/CAN_L) to the module
  • Failed running board control module (internal fault)
  • Gateway/module addressing or programming error after module replacement or update

Symptoms

  • Running board(s) fail to deploy or retract or operate intermittently
  • Instrument cluster or message center may show a body control / running board fault or ‘Service Running Board’ message
  • U0251 stored and RBM-B not listed/present in module list of scan tool
  • Related body functions on same network may operate erratically
  • No serial data or intermittent data from RBM-B when monitoring network traffic

What to check

  • Connect a tech-level scan tool and confirm U0251 and any other U-codes; note freeze frame and occurrence rate
  • Confirm RBM-B is (or is not) present in the tool’s module list or network scan
  • Visually inspect RBM-B connector and harness for corrosion, water intrusion, rodent damage or broken wires
  • Check fuses and relays feeding the running board control module; verify battery voltage at fuse with key on and engine off
  • Backprobe module connector for key-on battery voltage and good ground at pinned power/ground terminals
  • Monitor CAN network with a scan tool: request data from RBM-B and check for responses or timeouts

Signal parameters

  • Typical CAN bus idle (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both near mid-rail)
  • Typical CAN bus dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
  • Expected bus differential when dominant ≈ 1.5–2.5 V
  • Termination: approx. 60 Ω between CAN_H and CAN_L across the network (two 120 Ω resistors in parallel)
  • Common bus data rates on vehicles: 500 kb/s or 250 kb/s — confirm network speed with service information

Diagnostic algorithm

  1. Safety first: secure vehicle, battery stable and vehicle in park; use recommended scan tool and wiring diagrams
  2. Read and record all stored U- and P-codes and freeze frame data. Clear codes and attempt to reproduce to confirm persistence.
  3. Confirm RBM-B presence: use scan tool to list modules on CAN network. If RBM-B absent, note if RBM-A or gateway present.
  4. Visually inspect RBM-B connector and harness at the running board for damage, corrosion, or a disconnected plug.
  5. Check power/ground: with key ON, verify battery voltage at RBM-B power pin and verify low-resistance ground to chassis. Check related fuses/relays.
  6. Check CAN wiring: measure CAN_H and CAN_L voltages at RBM-B. Compare to expected (see signal_params). Measure resistance between H and L (~60 Ω).
  7. Use an oscilloscope or scope-enabled scan tool to view CAN signals while attempting communication; look for missing frames, dominant bus held low/high, or noise.
  8. If CAN signals are good at RBM-B connector but module does not respond, check for short between module and other nodes by disconnecting RBM-B and measuring bus behavior; reconnect and observe.
  9. Inspect other modules on same network for errors and verify gateway operation. If only RBM-B affected, suspect module or harness local to RBM-B.
  10. Repair identified wiring/connectors, replace blown fuse/relay, clean/repair corrosion, or repair damaged harness per wiring diagrams.
  11. If wiring and power/ground confirmed good and module still doesn’t respond, consider replacing or reflashing RBM-B only after confirming correct part number, coding and immobilizer/gateway procedures per dealer information.
  12. After repair, clear codes and verify operation and that code does not return during road test and repeated power cycles.

Likely causes

  • Connector disconnected or corroded at RBM-B
  • Power or ground fault to RBM-B (blown fuse, open circuit)
  • Damaged or shorted CAN pair between RBM-B and gateway/ECU
  • RBM-B internal failure or corrupted software after replacement

Fault status

⚠️ Status
Lost communication with Running Board Control Module 'B' — RBM-B not responding on vehicle network (CAN). Module absent or no serial data received.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5-3.0 hours

Similar codes

U0100 U0121 U0140 U0155 U0250 U0252
371

Browse 371 HYUNDAI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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