Fix U0129: Mechanic’s solutions that WORKED

Code U0129: Lost Communication with Brake System Control Module points to a loss of communication with the Brake System Control Module. This can be a serious issue as it affects the vehicle’s ability to communicate with a critical safety system. It may lead to problems with the brakes, ABS, and traction control.

U0129 quick overview

Meaning	U0129: Lost Communication with Brake System Control Module
Is it serious?	Yes, it can affect the vehicle’s safety and performance.
Possible causes	– Wiring issues (damage, corrosion, loose connections) – Faulty Brake System Control Module – Low battery voltage – Problems with the Controller Area Network (CAN) bus
How to diagnose?	– Check for physical damage or rubbing on wiring, especially near the Brake System Control Module. – Inspect wiring and connectors for corrosion or loose connections. – Test battery and charging system to ensure proper voltage. – Use a compatible scanner to verify communication with the Brake System Control Module. – Check the power supply to the Brake System Control Module. – Review any relevant Technical Service Bulletins (TSBs) for known issues.

To diagnose this code, start by inspecting the wiring for damage, corrosion, or loose connections, especially near the Brake System Control Module. Test the battery and charging system to ensure they provide the correct voltage. Use a compatible scanner to check communication with the module and verify the power supply.

How to fix U0129 – learn from mechanics

I looked into U0129 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Toyota

Vehicle Information: 2010 Toyota Tundra, 4.6L Engine.

Issue + Repair History:

• The vehicle had a dead battery, customer jumped it from another vehicle.
• After running, dashboard lights for ABS, Traction control, brake, power steering, battery, MIL on, and Hi/lo 4×4 flashing.
• Scanned code U0129 lost communication with brake system control module.
• Replaced the battery, but the issue persisted; the vehicle drives okay with no 4×4 operation.

Other OBD2 Codes + Test Results:

• Code U0129 scanned.

Suggestions to Fix Issue:

• Check using a scan tool for zero point calibration data.
• Examine the CAN communication line for shorts.
• Measure resistance at DLC3 pins 6 and 14.
• Disconnect the negative battery cable and touch it to the positive terminal for 30 seconds to drain capacitors in ECUs.
• Zero point calibration of the yaw rate sensor can be done at the check connector.

Final Fix:

• Awaiting dealer to flash a new ECM and send it out; suspecting ECM as the shorted component.

Case #2: Toyota

Vehicle Information: 2009 Toyota Camry XLE, 2.4L Engine.

Issue + Repair History:

• Customer reported Red “Brake” and Yellow “ABS” lights on.
• DTC set: U0129 Loss of Communication with ABS Skid Control Actuator.
• Speedometer reads 0 when lights are on; otherwise, it functions normally.
• When lights are off, ABS Module communicates with no DTCs.
• No communication with ABS Module when lights are on.
• Voltage readings: CAN H at 3.30V and CAN L at 3.20V; changes to 2.57V and 2.43V respectively when ABS Skid Control Actuator is unplugged.
• Tested Powers & Grounds at the Anti-Skid Control Actuator Connector; all readings normal.
• The vehicle has been serviced regularly by the technician.

Other OBD2 Codes + Test Results:

• U0129 in the Instrument Panel Control Module.

Suggestions to Fix Issue:

• Perform a detailed diagnostic, possibly involving a scan tool.
• Consider reaching out to UP FIX for module repair.
• Check for intermittent issues that may indicate a faulty module.
• The module should be plug-and-play; no reflashing required.
• Consider a “wiggle test” to check for intermittent connection issues.

Final Fix:

• Replaced the Anti-Skid Module with a new OEM Toyota part.
• No programming/flashing required for the new module.
• Issue resolved with the new Anti-Skid Module installation.

Case #3: Toyota

Vehicle Information: 2013 Toyota Sienna LE, 3.5L Engine.

Issue + Repair History:

• Vehicle drove a lot on gravel roads.
• New symptom: No speedometer or odometer after driving over a railway crossing.
• A new fuse (ECU IG 1) in the left IP panel fuse block was blown and replaced.
• Warning lights came on after hitting ramps of a 4-post lift.
• Voltage readings and fuse checks were normal.
• Load tested and back pinned VSC module connector with no success.
• Possible damage to the Power Control module (BCM on left IP panel).
• Previous issue: Power functions not working in rear door, both sliding doors, no power windows. MIL/SRS/Trac/VSC/BASF lights on. Found and replaced blown ECU B fuse.

Other OBD2 Codes + Test Results:

• Initially: ABS- C1432, SRS- B1650, OCC- B1794, BCM- B1242, U0201, U0202, U1002.
• Later: ECM- P0500, P1603, SRS- B1650, BCM- B2324, U0201, U0202, U1002, IPC-U0129, EMPS-U0129.
• No communication with ABS/VSC module when the ECU IG 1 fuse is powered.

Suggestions to Fix Issue:

• Check wiring harness for loose connections or short/open circuits, especially in usual rubbing points.
• IGN 1 relay, controlled by the Power Management Control ECU, might be causing issues.
• Consider replacing the ABS VSC module; a used module might not require reprogramming.

Final Fix:

• Replaced the ABS VSC module with a used one, as it doesn’t store a VIN and doesn’t require reprogramming.
• After the replacement, the vehicle drove many miles without failure.
• The initial ECU B fuse blowing issue remains unexplained but hasn’t reoccurred.

Case #4: Toyota

Vehicle Information: 2005 Toyota Prius, 1.5L Engine, 142,724 miles.

Issue + Repair History:

• Initially came with a P0A7F code (Battery pack deteriorating).
• Light collision repair to passenger front corner.
• When interfacing with Techstream, all dashboard lights would turn on and flash, vehicle would shut down. No communication with Techstream.
• Generic and Snap-on Solus Ultra scanners showed multiple CAN-bus issues (U0073, U0121, U0146, U0129, U0111, U0100, V0293, P0A1D).
• Resistance readings at DLC3 connector mostly within specs except for signal and chassis ground being slightly high.
• Suspected damage to wiring or control module.

Other OBD2 Codes + Test Results:

• Various communication loss codes (U0073, U0121, U0146, U0129, U0111, U0100, V0293, P0A1D).
• P0500 (VSS), P1603 (Engine stall history), SRS- B1650, BCM- B2324, U0201, U0202, U1002, IPC-U0129, EMPS-U0129.
• No communication with ABS/VSC module when ECU IG 1 fuse powered.

Suggestions to Fix Issue:

• Check for a bad 12-volt battery, load test.
• Inspect fuse box/harness in LF for damage or corrosion.
• Verify correct setup in Techstream, including interface configuration.
• Measure CAN bus resistance at DLC3 connector.
• Check for water in connectors causing shorts.
• Inspect auxiliary battery ground for proper connection.
• Consider testing or replacing individual cells in the HV battery.

Final Fix:

• Found a 10K ohm short from B+ve pin 16 to Can bus Hi & Can bus Lo in DLC connector.
• Cleaned up pins in the DLC connector, restoring communication with Techstream.
• Replaced HV battery, addressing the P0A7F code and resolving the issue.
• Techstream & Fluke essential for diagnosing Prius issues.

Case #5: GMC

Vehicle Information: 2010 GMC Yukon XL 1500 Denali, 6.2L Engine

Issue + Repair History:

• Intermittent issue: Stabilitrak light and ABS light come on, door locks cycle constantly.
• Radio goes mute followed by Stabilitrak light coming on and door locks cycling.
• Inspected data bus wiring for potential rubbing damage; none found.
• Difficult to diagnose due to the intermittent nature of the issue.
• No useful information captured during driving with a Tech2 scanner.
• Park assist module code for low voltage.

Other OBD2 Codes + Test Results:

• No specific additional OBD2 codes mentioned.
• Power and grounds to ABS module tested and found okay.

Suggestions to Fix Issue:

• Check for UXXX codes in all systems, especially the BCM (Body Control Module), as it is the network hub.
• Physically remove and clean the main grounds at the engine and body, even if testing at the modules shows they are okay.
• Snapshot BCM and ABS data with Tech2 during the issue to identify any abnormal data inputs.
• Look into a potential bulletin regarding CAN or GMLAN short circuits, which could cause similar issues.

Final Fix:

• Cleaned battery to engine, battery to frame, and battery to body cables, and added dielectrical grease.
• Issue resolved after performing these maintenance steps.

Advice	Brief Description of Diagnostic Step
Check CAN Bus Resistance	Measure resistance between CAN bus lines at the DLC connector; typically should be around 60 ohms.
Inspect Wiring and Connectors	Thoroughly inspect wiring and connectors for damage, corrosion, or loose connections.
Check for Ground Issues	Verify the condition and connections of ground wires, especially main battery grounds.
Examine Battery and Charging System	Test the vehicle’s battery and charging system for proper operation and voltage levels.
Perform Complete Vehicle DTC Scan	Scan all vehicle modules for any related Diagnostic Trouble Codes.
Check for Communication with Modules	Verify communication with critical modules, such as ABS, using a compatible scanner.
Test Power Supply to Modules	Ensure modules like the ABS control module are receiving the correct power supply.
Investigate Physical Damage or Rubbing	Check for physical damage or wear on wiring, particularly in areas prone to abrasion.
Consider Module Repair or Replacement	If diagnostics indicate a module fault, consider repairing or replacing the affected module.
Review Relevant Technical Bulletins	Look up any Technical Service Bulletins related to communication or module issues for the specific vehicle model.