U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module

What it means

The CAN messages from the ABS module are missing. ABS, ESP/ESC and traction control all become inactive (warning lamps illuminate), the speedometer may go to zero on cars that derive vehicle speed from the ABS module, and adaptive cruise control is disabled.

At a glance

• Usually lights the warning light of the system that lost communication

How it works

The ABS control module monitors wheel-speed sensors and controls anti-lock braking, and it shares status and vehicle data with other modules over the CAN network. U0121 sets in a module that stops receiving expected ABS messages; it does not by itself prove the ABS module has failed.

Common causes

Most likely first.

1. ABS module unpowered (often a corroded ground or fuse)
2. CAN wiring to ABS broken
3. ABS module failed

Symptoms

• ABS or brake warning light
• ABS, stability control, or traction control may be unavailable
• Other communication codes

How to diagnose it

1. Scan all modules for U0121 and related power, ground, or network codes, and identify whether the ABS module responds to the scan tool.
2. Check battery voltage and the ABS module’s fuses, since a loss of its supply can make its network messages disappear.
3. Inspect ABS module grounds, connectors, and nearby CAN wiring for corrosion, water, looseness, or physical damage.
4. Use the vehicle wiring diagram to test ABS module power and ground under the manufacturer's specified conditions.
5. If the module is powered and grounded, test the CAN circuit between it and the network as specified before concluding the module itself is faulty.

Common fixes

• Restore a poor ABS module ground or power connection.
• Replace a failed ABS fuse after checking for the underlying cause.
• Repair damaged CAN wiring or clean and secure corroded connectors.
• Replace or reconfigure the ABS module only if network and supply tests support that diagnosis.

Can I keep driving?

Base braking may still work, but ABS and possibly stability or traction control can be unavailable; the speedometer or other functions may also be affected. Drive cautiously, leave extra stopping distance, and avoid slippery roads or hard braking until the fault is repaired.