U0129

Code

HYUNDAI U — Network/User

Lost Communication With Brake System Control Module | Lost Communication With Brake System Control Module (C-CAN)

Brand: HYUNDAI

Type: U — Network/User

Views: UK: 9 EN: 20 RU: 7

AI status

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Page language: EN

Causes

Open, shorted or damaged CAN bus wiring (C-CAN) between brake module and rest of network
Poor module power supply or ground (fuse, relay, battery connections)
Damaged or corroded module connector or pin(s)
Missing or failed CAN termination resistor
Faulty Brake System Control Module (ABS/ESC module)
Intermittent connection caused by chafing, water intrusion or broken wire

Symptoms

ABS, ESC or BRAKE warning lamp(s) illuminated
Loss of ABS/ESC functionality (basic hydraulic brakes typically still work)
Traction control/ESP warning lights
Other modules showing related communication DTCs
Possible stored freeze frame data and intermittent faults

What to check

Use a scan tool to confirm U0129 and read freeze frame and related codes
Check for presence of other network errors or modules not responding
Verify battery voltage and ignition ON power to brake module circuits
Inspect fuses and relays feeding the brake control module
Visually inspect brake module connector and harness for damage or corrosion
Measure CAN_H and CAN_L voltages and termination resistance

Signal parameters

CAN bus idle (recessive): approximately 2.5 V on CAN_H and CAN_L (both similar)
CAN bus dominant: CAN_H rises ~3.5 V and CAN_L falls ~1.5 V (vehicle dependent)
Termination resistance: approx. 60 ohms between CAN_H and CAN_L (two 120Ω in parallel)
Typical bus data rates: commonly 250 or 500 kbps (confirm vehicle specification)
Expected behavior: periodic messages from brake/ABS module at regular intervals (frequency depends on message ID)

Diagnostic algorithm

Connect a capable scan tool and confirm U0129; record freeze frame and note other U-codes present.
Cycle ignition power (OFF/ON) and see if code returns immediately or intermittently.
Check battery voltage and charging system; low voltage can cause network faults.
Verify fuses/relays and power/ground at the brake system control module connector (voltage with key ON and engine off as required).
Visually inspect the brake module connector and nearby harness for corrosion, bent pins, water intrusion, or physical damage.
With ignition ON, measure CAN_H and CAN_L at the module connector: confirm recessive voltages ~2.5 V and that both lines move to dominant when bus activity occurs.
Measure termination resistance across CAN_H and CAN_L (vehicle off): expect ~60 Ω. An open or very low resistance indicates missing or shorted termination.
Use an oscilloscope or advanced CAN tester to confirm valid CAN frames and to identify noise, shorts to battery/ground, or missing messages from the brake module.
Perform a wiggle test of the harness and connectors while monitoring the bus/tools for interruptions or code reappearance; inspect known pinch points (doors, suspension, engine bay).
If wiring checks out, attempt to communicate directly with the brake module (diagnostic service tool) or substitute a known-good module per OEM procedure if available.
Repair or replace wiring, connectors, or module as indicated by diagnostics. Clear codes and perform road test and network re-check to confirm repair.
If intermittent and unable to reproduce, monitor vehicle and capture data during occurrence or use a data logger.

Likely causes

Blown fuse or relay supplying brake module power/ignition
Corroded connector at brake control module or junction block
Short to battery or ground on CAN_H or CAN_L conductors
Open in CAN_H or CAN_L wiring (broken harness near suspension/engine bay)
Failed brake/ABS module electronics

Fault status

⚠️ Status

Communication with Brake System Control Module not established on C‑CAN network — module messages not received by other controllers.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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U0129 — Lost Communication With Brake System Control Module | Lost Communication With Brake System Control Module (C-CAN)