A blown fuse or failed relay in the ABS module power circuit, including the underhood fuse block and the module's own internal supply.; Corroded, loose, or damaged connectors at the ABS module, especially where road salt and moisture attack the pins.; An open or shorted CAN bus wire between the ABS module and the network gateway or another controller.; A failed ABS control module, often from internal solder joint fatigue or water intrusion into the electronics housing.; A weak or failing battery or charging system that drops network voltage below the threshold the ABS module needs to stay online.; A poor ground connection at the ABS module or the chassis point it shares with other electronics.; Another module on the same CAN segment holding the bus down and preventing ABS messages from getting through.

DTC codes / U01xx · Lost communication with a control module / U0121

U0121Lost Communication With Anti-Lock Brake System (ABS) Control Module

U0121 is a communication fault where the engine control module or another network controller has stopped receiving messages from the anti-lock brake system (ABS) control module over the vehicle's controller area network (CAN) bus. The code means the ABS module is either not powered, not communicating, or the wiring between it and the rest of the network is compromised, so the computer flags a lost node rather than a brake hydraulic fault.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaLost communication with a control module

Symptoms you may notice

The ABS warning light, traction control light, and often the brake warning light illuminate together on the dashboard.
The stability control and hill-start assist systems may be disabled, with a message displayed in the driver information center.
The speedometer may drop out or read erratically because wheel speed data from the ABS module is no longer broadcast to the cluster.
The transmission may shift harshly or stay in a default gear if it relies on wheel speed input from the ABS module.
Often no symptoms besides the warning light, with normal brake pedal feel and normal base braking performance.

Common causes, most likely first

A blown fuse or failed relay in the ABS module power circuit, including the underhood fuse block and the module's own internal supply.
Corroded, loose, or damaged connectors at the ABS module, especially where road salt and moisture attack the pins.
An open or shorted CAN bus wire between the ABS module and the network gateway or another controller.
A failed ABS control module, often from internal solder joint fatigue or water intrusion into the electronics housing.
A weak or failing battery or charging system that drops network voltage below the threshold the ABS module needs to stay online.
A poor ground connection at the ABS module or the chassis point it shares with other electronics.
Another module on the same CAN segment holding the bus down and preventing ABS messages from getting through.

How to diagnose and fix it

Scan all modules for communication codes and record which controllers are online, then check the ABS module's power and ground at its connector with a multimeter before condemning any part.
Inspect and test the ABS fuses and relays in the underhood and interior fuse boxes, replacing any that are blown or showing high resistance.
Disconnect the ABS module connector and check for corrosion, pushed-back pins, or water intrusion, cleaning and reseating as needed.
Measure CAN bus resistance and voltage at the diagnostic connector and at the ABS module to confirm the bus is intact and not shorted.
Perform a voltage drop test on the ABS module ground circuit and repair any corroded or loose ground strap.
If power, ground, and bus integrity are confirmed, substitute a known-good ABS module or perform module programming and adaptation as required.
Clear the code and verify communication returns across all modules, then road test to confirm the ABS and stability systems operate normally.

What a lost ABS module really tells you

A U0121 often appears after a dead battery or jump-start because low voltage can drop the ABS module offline and set the code even though the module is healthy.
Technicians frequently replace the ABS module only to find a corroded ground or a chafed CAN wire was the real cause, so always verify power, ground, and bus integrity first.
On many vehicles the ABS module is the gateway for wheel speed data, so a U0121 can trigger unrelated transmission or speedometer complaints that disappear once communication is restored.
A single failed module on the CAN bus can pull the entire network down, so check whether other modules are also missing before assuming the ABS module itself is faulty.
After repairing the circuit, some vehicles require a relearn or initialization of the ABS module before the warning lights will clear, even when the fault is fully resolved.

Can I keep driving with U0121?

The vehicle usually remains safe to drive because base hydraulic braking is unaffected, but you should avoid relying on anti-lock braking, traction control, or stability assist until the fault is repaired. Drive conservatively in wet or slippery conditions and have the system diagnosed promptly, since the ABS and stability systems are disabled while the code is active.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.