U0129

Lost Communication With Brake System Control Module

Car Fault Codes Network

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 30 minutes – 6 hours

Typical cost SAR 100 to 8,000

What it means

A module on the network is no longer receiving messages from the brake system control module. This module differs by maker: on some vehicles it is the ABS module itself, while on hybrids and electric vehicles it may be the electronic brake unit that blends friction braking with regenerative braking. The driver usually loses ABS and stability control, and on some systems the brake pedal feel may change.

Symptoms5

Brake, ABS and stability warning lights on.
Brake system warning message in the cluster.
Changed brake pedal feel or longer stopping distance on some hybrids.
Cruise control or brake-based assist features disabled.
Scan tool unable to communicate with the brake module.

Possible causes6

Blown fuse for the brake module or its pump.
Weak battery or low voltage while running.
Corrosion or water in the module connector.
Poor module ground connection.
Open or shorted CAN wiring to the module.
Internal failure of the brake control module.

How to diagnose5

Check the service information to identify which module this code refers to on your vehicle, as brake module names vary between makers.
Read codes from all modules and check for ABS or other brake codes.
Try to connect directly to the module; if it does not respond, check its fuses, power and ground.
Inspect the module connector for water or corrosion.
Measure CAN line resistance and voltages and compare them with the manufacturer specification.

How to fix6

Replace the blown fuse and charge or replace the battery.
Clean the module connector and repair the ground point.
Repair damaged network wiring.
Update the module software if the manufacturer has an update.
Replace the module if proven faulty, then program it and perform brake bleeding and sensor calibration as instructed.
Clear codes in all modules, test the brakes carefully and confirm the lights stay off.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for a fuse, battery or connector, and reaches the upper end if the brake module or hydraulic unit must be replaced and programmed.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

Is this code dangerous?

Yes, because it involves the brake system. Basic braking usually still works, but ABS and stability control may be lost and some hybrids have a changed pedal feel. If the brakes feel weak, do not drive; have the car towed.

How is this different from U0121?

U0121 refers specifically to the ABS module, while this code refers to the brake control module in general. On some vehicles they are the same unit; on others, especially hybrids, they are separate.

This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

Report a mistake

Spotted something inaccurate? Tell us and we will review and correct it.