U0129Generic code

U0129 Code: Lost Communication With Brake System Control Module

The U0129 code means other modules in the vehicle stopped receiving messages from the brake system control module. The usual effect is that brake assist features are turned off. Start with the battery, the module power and grounds, and the network wiring before you replace anything.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$0 to $1,500
Difficulty
Medium
System
Communication Network
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the U0129 code means

The modules in your vehicle constantly exchange information over a data network, normally CAN. The brake system control module manages the brakes on vehicles with electronic braking. When the other modules stop receiving its messages for a set amount of time, they store the U0129 code.

The code is stored by the module that is missing the information, not by the module that is failing. That is why it often shows up in several modules at once. The problem can be in the brake system control module itself (no power, no ground or an internal failure), in its connector, or in the network wiring that runs to it.

Typical location: in the engine compartment. If that area is exposed to water or was worked on recently, start there.

Diagnostic order: 1) battery and alternator (low voltage is the number one cause of bursts of U codes); 2) the module list in the scan tool, to see which module is not responding; 3) power, fuses and grounds of the module that is not responding; 4) network resistance at the OBD2 connector (about 60 ohms between pins 6 and 14 with the battery disconnected); 5) module connectors and wiring, especially in damp areas (under carpets, in doors, in the trunk).

The CAN Network and U Codes guide explains in detail how to measure the network and isolate the module that is bringing it down.

U0129 code symptoms

  • Warning lights on in the dash.
  • Brake assist features turned off.
  • Communication codes stored in several modules at the same time.
  • The fault can be intermittent (for example, in rain or over bumps).

Likely causes of the U0129 code

  1. Module power or ground. A blown fuse, a corroded ground or a loose power connector.
  2. Weak battery or low voltage. Modules reset and lose communication.
  3. Module connector with water or corrosion. Very common on modules mounted in damp areas.
  4. Damaged network wiring. A CAN high or CAN low wire that is cut or shorted.
  5. Failure of the brake system control module. Only after everything above has been ruled out.
  6. Poorly installed accessory. Alarms or trackers wired into the network.

How to diagnose the U0129 code step by step

  1. Measure the battery voltage at rest (12.4 to 12.7 V) and with the engine running (13.5 to 14.7 V).
  2. Check in the scan tool which modules respond. If the brake system control module does not show up, the fault is in it or in its connection.
  3. Check the module fuses and grounds using the wiring diagram.
  4. Measure the network at the OBD2 connector. About 60 ohms between pins 6 and 14 with the battery disconnected, and roughly 2.5 V on each line with the key on.
  5. Inspect the module connector for moisture, corrosion and bent pins.
  6. Measure CAN high and CAN low at the module connector. They should have continuity back to the OBD2 connector.

How to fix the U0129 code

  • Repair the module power, ground or connector.
  • Repair the network wiring.
  • Replace and program the module if it has failed (it almost always needs programming).
  • Clear the codes in all modules and verify the repair.

Technical reference data

  • CAN network resistance between OBD2 pins 6 and 14: about 60 ohms.
  • CAN high: 2.5 to 3.5 V. CAN low: 1.5 to 2.5 V.
  • These are reference values only: check the manufacturer data for your vehicle.

Brand-specific notes

After the repair, clear the codes in every module. Many of them keep old U codes from the time the fault was present, and those can leave a warning light on or confuse the next scan.

Accessories installed after purchase (alarms, GPS trackers and OBD2 adapters left plugged in all the time) are a frequent cause of network problems, so unplug them first when you chase a U0129 code.

Frequently asked questions about U0129

Can I drive with a U0129 code?
It depends on the module. This one is tied to the brakes, so drive with caution and get the vehicle to a shop soon, because assist features may be unavailable.
Does U0129 mean the brake system control module is bad?
Not necessarily. The most frequent causes are its power supply, its ground, its connector or the battery, and the module itself is replaced last.
Why are so many codes showing at once?
Every module that was waiting for information from the affected module stores its own fault. One problem on the network can therefore produce a long list of codes.
How much does it cost to fix U0129?
From $0, if it was only the battery or a fuse, up to about $1,500 if the module has to be replaced and programmed.
What does the U0129 code mean?
It is a generic OBD2 network code that means the vehicle lost communication with the brake system control module. It points to a power, ground, connector or wiring problem as often as to the module itself.
Will a U0129 code clear itself?
If the cause was a brief voltage dip, the code may stop returning after the fault goes away, but the stored code stays until it is cleared. A loose connector or a damaged wire will bring it back.
What is the most common cause of U0129?
Low battery voltage and problems with the module power, ground or connector are the most common causes. Check those before you suspect the module.

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