U0121

Lost Communication With Anti-Lock Brake System (ABS) 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

Vehicle modules share data over a network such as the CAN bus. This code is set in another module, such as the engine computer (ECU) or the instrument cluster, when messages from the ABS module stop arriving. Normal braking still works, but ABS and stability control are usually disabled until the fault is fixed.

Symptoms5

ABS and stability or traction warning lights on in the cluster.
Wheels can lock under hard braking because ABS is not working.
Speedometer dead or intermittent on vehicles that get speed from the ABS module.
Transmission in limp mode or with changed shifting on some vehicles.
Scan tool unable to communicate with the ABS module.

Possible causes6

Blown ABS power fuse; the unit usually has a large fuse for the pump and a small one for the module.
Weak battery or loose terminals causing low voltage while running.
Corrosion or water in the ABS module connector, which usually sits in the engine bay exposed to heat and moisture.
Poor ABS module ground connection.
Open or shorted CAN wiring between the ABS module and the rest of the network.
Internal failure of the ABS control module.

How to diagnose6

Read codes from all modules; if several modules report this code while others still talk, the fault is likely at the ABS module itself.
Try to connect directly to the ABS module; if it does not respond, start with its power and ground.
Check both ABS fuses and measure power and ground at the module connector with the key on.
Disconnect the module connector and inspect it for water, corrosion or loose pins.
Measure CAN line resistance and voltages and compare them with the manufacturer specification to find an open or short.
Only condemn the module after power, ground and wiring are proven good.

How to fix6

Replace the blown fuse and find the cause if it blows again.
Charge or replace the battery, and clean and tighten its terminals.
Clean or repair the ABS module connector and repair the ground point.
Repair damaged CAN wiring to the module.
Have the electronic module repaired by a specialist or replace it, then program it and perform a brake bleed procedure if the manufacturer requires one.
Clear codes in all modules, road test and confirm the ABS light stays 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 corroded connector, and reaches the upper end if the ABS 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

Do the brakes still work with this code?

Yes, normal braking still works, but ABS and stability control do not, so the wheels can lock in a panic stop, especially on wet or sandy roads. Drive carefully and leave more distance.

Why did my speedometer stop with this code?

On many vehicles the ABS module reads the wheel speed sensors and sends vehicle speed to the cluster and transmission over the network. When it drops off, the speedometer stops.

Does this code mean the ABS module is bad?

Not necessarily. Most cases come from a fuse, a water-damaged connector or a poor ground, and these should be checked before replacing the expensive module.

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.