Code
U0129
Generic
U — Network/User
Lost Communication With Brake System Control Module A
Views:
UK: 19
EN: 25
RU: 43
AI status
Completed
Completed
100%
Causes
- Brake System Control Module (ABS/Brake ECU) fault or internal failure
- Open, short, or high resistance in CAN bus wiring (CAN High / CAN Low) to the brake module
- Corroded, loose, or damaged connector or pin at the brake module
- No power or poor ground to the brake module (fuse, relay, wiring)
- Missing or incorrect network termination (faulty/missing 120 Ω resistors or parallel resistance)
- Other control module on same CAN bus pulling the network down
Symptoms
- ABS, ESC/TRAC, and/or Brake warning lights illuminated
- Reduced or disabled ABS/traction control functions
- Loss of other features that rely on brake module data (e.g., stability control, regenerative braking coordination)
- Multiple network communication DTCs present (other U-codes)
- Possible limp-home or reduced vehicle performance modes
What to check
- Read and record all stored U-codes and freeze frame data with a full-function scan tool
- Check battery voltage and health; low battery can cause communications/errors
- Visually inspect brake module connector for corrosion, bent pins, moisture, or damage
- Verify fuses and relays that supply the brake module are intact and have correct voltage
- Check module ground(s) for low resistance to chassis/negative battery
- Scan for presence of the brake module on the network (module list or CAN node scan)
Signal parameters
- CAN bus idle voltages (measured to chassis): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each), with differential ≈ 0 V at recessive state
- CAN bus dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active bits
- Typical CAN bus differential impedance: ~60 Ω measured across CAN_H and CAN_L (two 120 Ω terminators in parallel)
- Common data rates: 500 kbps (high-speed CAN) or 250 kbps / 125 kbps (medium/low-speed systems) — confirm vehicle-specific rate
- Expected steady waveform with clean transitions; excessive noise, clipped edges, or missing frames indicate bus fault
Diagnostic algorithm
- Confirm the DTC: Use scan tool to retrieve U0129 and any related codes. Note freeze frame and occurrence conditions.
- Check vehicle power: Measure battery voltage at rest and while cranking; verify fuses and relays that feed the brake module have voltage.
- Inspect connectors: Physically inspect and reconnect the brake module connector(s). Look for corrosion, bent pins, water intrusion, or damage.
- Verify ground: Measure resistance from module ground to chassis/negative battery. Expect very low resistance (
- Check for module presence: Use a scan tool to see if the brake module is listed/responding. If not present, proceed with bus-level checks.
- Verify CAN voltages at the module connector: With key ON, measure CAN_H and CAN_L to chassis and verify idle and dominant voltages match expected parameters.
- Compare bus at another known-good node: Probe CAN lines at the gateway/ECM or instrument cluster to see if messaging is present on the rest of the bus.
- Oscilloscope check: Use an oscilloscope to view waveform quality at the module and at multiple nodes to identify noise, reflections, or missing frames.
- Isolate suspect wiring: Disconnect downstream modules or sections of the bus as appropriate to isolate the fault. Check continuity and shorts between pins.
- Repair as indicated: Repair damaged wiring or connectors, replace corroded terminals, correct grounding/power faults.
- Module replacement/reprogramming: If wiring and power/ground are good and the module does not respond, replacement and proper programming may be required.
- Clear codes and test: Clear DTCs and perform a functional test / road test to confirm the fault does not return.
Likely causes
- Blown fuse or poor ground to the brake module preventing it from powering up
- Damaged or disconnected connector at the brake module
- Short or open in CAN High/CAN Low wires between brake module and gateway/PCM
- Faulty ABS/Brake module hardware
- Another module on the same CAN bus causing bus contention
Fault status
Status
Lost Communication With Brake System Control Module A — CAN network messages from the brake control module are not being received by other control modules. Investigate power, ground, connectors, wiring, termination, and module response.
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
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Code
U0129
HYUNDAI
U — Network/User
Lost Communication With Brake System Control Module | Lost Communication With Brake System Control Module (C-CAN)
Views:
UK: 9
EN: 20
RU: 7
AI status
Completed
Completed
100%
Causes
- Brake System Control Module (ABS/Brake ECU) fault or internal failure
- Open, short, or high resistance in CAN bus wiring (CAN High / CAN Low) to the brake module
- Corroded, loose, or damaged connector or pin at the brake module
- No power or poor ground to the brake module (fuse, relay, wiring)
- Missing or incorrect network termination (faulty/missing 120 Ω resistors or parallel resistance)
- Other control module on same CAN bus pulling the network down
Symptoms
- ABS, ESC/TRAC, and/or Brake warning lights illuminated
- Reduced or disabled ABS/traction control functions
- Loss of other features that rely on brake module data (e.g., stability control, regenerative braking coordination)
- Multiple network communication DTCs present (other U-codes)
- Possible limp-home or reduced vehicle performance modes
What to check
- Read and record all stored U-codes and freeze frame data with a full-function scan tool
- Check battery voltage and health; low battery can cause communications/errors
- Visually inspect brake module connector for corrosion, bent pins, moisture, or damage
- Verify fuses and relays that supply the brake module are intact and have correct voltage
- Check module ground(s) for low resistance to chassis/negative battery
- Scan for presence of the brake module on the network (module list or CAN node scan)
Signal parameters
- CAN bus idle voltages (measured to chassis): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each), with differential ≈ 0 V at recessive state
- CAN bus dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active bits
- Typical CAN bus differential impedance: ~60 Ω measured across CAN_H and CAN_L (two 120 Ω terminators in parallel)
- Common data rates: 500 kbps (high-speed CAN) or 250 kbps / 125 kbps (medium/low-speed systems) — confirm vehicle-specific rate
- Expected steady waveform with clean transitions; excessive noise, clipped edges, or missing frames indicate bus fault
Diagnostic algorithm
- Confirm the DTC: Use scan tool to retrieve U0129 and any related codes. Note freeze frame and occurrence conditions.
- Check vehicle power: Measure battery voltage at rest and while cranking; verify fuses and relays that feed the brake module have voltage.
- Inspect connectors: Physically inspect and reconnect the brake module connector(s). Look for corrosion, bent pins, water intrusion, or damage.
- Verify ground: Measure resistance from module ground to chassis/negative battery. Expect very low resistance (
- Check for module presence: Use a scan tool to see if the brake module is listed/responding. If not present, proceed with bus-level checks.
- Verify CAN voltages at the module connector: With key ON, measure CAN_H and CAN_L to chassis and verify idle and dominant voltages match expected parameters.
- Compare bus at another known-good node: Probe CAN lines at the gateway/ECM or instrument cluster to see if messaging is present on the rest of the bus.
- Oscilloscope check: Use an oscilloscope to view waveform quality at the module and at multiple nodes to identify noise, reflections, or missing frames.
- Isolate suspect wiring: Disconnect downstream modules or sections of the bus as appropriate to isolate the fault. Check continuity and shorts between pins.
- Repair as indicated: Repair damaged wiring or connectors, replace corroded terminals, correct grounding/power faults.
- Module replacement/reprogramming: If wiring and power/ground are good and the module does not respond, replacement and proper programming may be required.
- Clear codes and test: Clear DTCs and perform a functional test / road test to confirm the fault does not return.
Likely causes
- Blown fuse or poor ground to the brake module preventing it from powering up
- Damaged or disconnected connector at the brake module
- Short or open in CAN High/CAN Low wires between brake module and gateway/PCM
- Faulty ABS/Brake module hardware
- Another module on the same CAN bus causing bus contention
Fault status
Status
Lost Communication With Brake System Control Module A — CAN network messages from the brake control module are not being received by other control modules. Investigate power, ground, connectors, wiring, termination, and module response.
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
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Send to email
Code
U0129
LAND ROVER
U — Network/User
Lost communication with the brake system control module
Views:
UK: 1
EN: 5
RU: 1
AI status
Completed
Completed
100%
Causes
- Brake System Control Module (ABS/Brake ECU) fault or internal failure
- Open, short, or high resistance in CAN bus wiring (CAN High / CAN Low) to the brake module
- Corroded, loose, or damaged connector or pin at the brake module
- No power or poor ground to the brake module (fuse, relay, wiring)
- Missing or incorrect network termination (faulty/missing 120 Ω resistors or parallel resistance)
- Other control module on same CAN bus pulling the network down
Symptoms
- ABS, ESC/TRAC, and/or Brake warning lights illuminated
- Reduced or disabled ABS/traction control functions
- Loss of other features that rely on brake module data (e.g., stability control, regenerative braking coordination)
- Multiple network communication DTCs present (other U-codes)
- Possible limp-home or reduced vehicle performance modes
What to check
- Read and record all stored U-codes and freeze frame data with a full-function scan tool
- Check battery voltage and health; low battery can cause communications/errors
- Visually inspect brake module connector for corrosion, bent pins, moisture, or damage
- Verify fuses and relays that supply the brake module are intact and have correct voltage
- Check module ground(s) for low resistance to chassis/negative battery
- Scan for presence of the brake module on the network (module list or CAN node scan)
Signal parameters
- CAN bus idle voltages (measured to chassis): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each), with differential ≈ 0 V at recessive state
- CAN bus dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active bits
- Typical CAN bus differential impedance: ~60 Ω measured across CAN_H and CAN_L (two 120 Ω terminators in parallel)
- Common data rates: 500 kbps (high-speed CAN) or 250 kbps / 125 kbps (medium/low-speed systems) — confirm vehicle-specific rate
- Expected steady waveform with clean transitions; excessive noise, clipped edges, or missing frames indicate bus fault
Diagnostic algorithm
- Confirm the DTC: Use scan tool to retrieve U0129 and any related codes. Note freeze frame and occurrence conditions.
- Check vehicle power: Measure battery voltage at rest and while cranking; verify fuses and relays that feed the brake module have voltage.
- Inspect connectors: Physically inspect and reconnect the brake module connector(s). Look for corrosion, bent pins, water intrusion, or damage.
- Verify ground: Measure resistance from module ground to chassis/negative battery. Expect very low resistance (
- Check for module presence: Use a scan tool to see if the brake module is listed/responding. If not present, proceed with bus-level checks.
- Verify CAN voltages at the module connector: With key ON, measure CAN_H and CAN_L to chassis and verify idle and dominant voltages match expected parameters.
- Compare bus at another known-good node: Probe CAN lines at the gateway/ECM or instrument cluster to see if messaging is present on the rest of the bus.
- Oscilloscope check: Use an oscilloscope to view waveform quality at the module and at multiple nodes to identify noise, reflections, or missing frames.
- Isolate suspect wiring: Disconnect downstream modules or sections of the bus as appropriate to isolate the fault. Check continuity and shorts between pins.
- Repair as indicated: Repair damaged wiring or connectors, replace corroded terminals, correct grounding/power faults.
- Module replacement/reprogramming: If wiring and power/ground are good and the module does not respond, replacement and proper programming may be required.
- Clear codes and test: Clear DTCs and perform a functional test / road test to confirm the fault does not return.
Likely causes
- Blown fuse or poor ground to the brake module preventing it from powering up
- Damaged or disconnected connector at the brake module
- Short or open in CAN High/CAN Low wires between brake module and gateway/PCM
- Faulty ABS/Brake module hardware
- Another module on the same CAN bus causing bus contention
Fault status
Status
Lost Communication With Brake System Control Module A — CAN network messages from the brake control module are not being received by other control modules. Investigate power, ground, connectors, wiring, termination, and module response.
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
Similar codes
Workshop Manuals
Repair manuals for LAND ROVER
3
Land Rover Defender 300Tdi — Workshop Manual (1996 model year)
Workshop Manual199,00 UAH
Land Rover Defender Workshop Manual Supplement & Body Repair Manual (1999 & 2002 MY)
Workshop Manual199,00 UAH
Land Rover Range Rover — Electrical Library (LRL 0453ENG, 2002)
Workshop Manual199,00 UAH
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