Loss of power or ground to the ABS module, such as a blown fuse or dirty, loose ground; OBD-Codes.com says this is the most common failure. An open in the CAN bus high (+) or low (-) wire (OBD-Codes.com, RepairPal). A short to power or to ground in either CAN bus wire (OBD-Codes.com). A weak or dead battery (RepairPal). A problem with the ABS module circuit, such as damaged or corroded wiring and connectors (OBD-Codes.com, RepairPal). A faulty ABS module or module software; OBD-Codes.com says a bad module is rare, while RepairPal lists a faulty ABS module as a common cause.

Charge or replace a weak battery (RepairPal). Replace a blown ABS module fuse or clean and tighten a bad ground (OBD-Codes.com). Clean corroded ABS module connector terminals and repair damaged wiring (OBD-Codes.com, RepairPal). Repair an open or shorted CAN bus wire (OBD-Codes.com, RepairPal). Reprogram the ABS module software, which RepairPal says can sometimes avoid a replacement. Replace the ABS module only after power, ground and network checks pass; OBD-Codes.com says most must be programmed or calibrated to the vehicle.

U0121🔧 Vehicle network (CAN bus) and anti-lock brake computerDrive with caution, fix soon

U0121: Lost Communication With ABS Control Module (Meaning, Causes, Fixes)

Q: What does U0121 mean?

U0121 means other modules on the car's network have lost communication with the anti-lock brake system (ABS) control module. Common causes are a loss of power or ground to the ABS module, an open or short in the CAN bus wiring, a weak or dead battery, or, less often, a faulty ABS module. The ABS control module is the computer that monitors and controls the anti-lock brakes. It reads the wheel speed sensors and operates valves in the hydraulic modulator to keep the wheels from locking up in a hard stop (RepairPal), and some makers call it the Electronic Brake Control Module (EBM). It shares information with the car's other computers over a two-wire network called the controller area network (CAN bus). U0121 sets when the ABS module is not receiving or sending messages on that network. OBD-Codes.com calls it a generic communication code that applies to most makes, including Mazda, Chevrolet, Dodge, VW, Ford, Jeep and GMC, with troubleshooting that varies by manufacturer and wiring. A useful first check from OBD-Codes.com: if a scan tool can still read codes from the ABS module, the fault is intermittent or a stored memory code; if it can't reach the ABS module at all, the problem is happening now. RepairPal notes that on many vehicles battery voltage must be between 9 and 16 volts for the code to set, and that if several modules are reporting lost communication the network itself is the likely problem. The sources differ a little on causes: OBD-Codes.com says the most common failure is loss of power or ground and a faulty module is rare, while RepairPal lists a dead battery, a faulty ABS module, the ABS module circuit and the CAN bus without ranking them, and adds that the module can sometimes be reprogrammed instead of replaced. Related codes include U0100 (lost communication with the engine computer) and U0101 (lost communication with the transmission computer).

Q: Can I keep driving with U0121?

Drive with caution, fix soon. Get it diagnosed soon. RepairPal rates U0121 Moderate, meaning it may not threaten safety right away. OBD-Codes.com gives no rating but lists the ABS light, and on some vehicles the traction control and stability control lights, coming on. When those lights are on, the anti-lock and stability features may not be working, so brake early, leave extra following distance and take extra care on wet or slippery roads until it is fixed.

Reviewed 2026-10-23 · Based on 2 cited source(s)

What U0121 means

The ABS control module is the computer that monitors and controls the anti-lock brakes. It reads the wheel speed sensors and operates valves in the hydraulic modulator to keep the wheels from locking up in a hard stop (RepairPal), and some makers call it the Electronic Brake Control Module (EBM). It shares information with the car's other computers over a two-wire network called the controller area network (CAN bus). U0121 sets when the ABS module is not receiving or sending messages on that network. OBD-Codes.com calls it a generic communication code that applies to most makes, including Mazda, Chevrolet, Dodge, VW, Ford, Jeep and GMC, with troubleshooting that varies by manufacturer and wiring. A useful first check from OBD-Codes.com: if a scan tool can still read codes from the ABS module, the fault is intermittent or a stored memory code; if it can't reach the ABS module at all, the problem is happening now. RepairPal notes that on many vehicles battery voltage must be between 9 and 16 volts for the code to set, and that if several modules are reporting lost communication the network itself is the likely problem. The sources differ a little on causes: OBD-Codes.com says the most common failure is loss of power or ground and a faulty module is rare, while RepairPal lists a dead battery, a faulty ABS module, the ABS module circuit and the CAN bus without ranking them, and adds that the module can sometimes be reprogrammed instead of replaced. Related codes include U0100 (lost communication with the engine computer) and U0101 (lost communication with the transmission computer).

Severity: can I keep driving?

Drive with caution, fix soon

Get it diagnosed soon. RepairPal rates U0121 Moderate, meaning it may not threaten safety right away. OBD-Codes.com gives no rating but lists the ABS light, and on some vehicles the traction control and stability control lights, coming on. When those lights are on, the anti-lock and stability features may not be working, so brake early, leave extra following distance and take extra care on wet or slippery roads until it is fixed.

Common symptoms

Check engine light on (OBD-Codes.com, RepairPal)
ABS warning light on (OBD-Codes.com, RepairPal)
Traction control (TRAC) light on, depending on the manufacturer (OBD-Codes.com)
Stability control (ESP/ESC) light on, depending on the manufacturer (OBD-Codes.com)
ABS-related performance problems (RepairPal)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Loss of power or ground to the ABS module, such as a blown fuse or dirty, loose ground; OBD-Codes.com says this is the most common failure
An open in the CAN bus high (+) or low (-) wire (OBD-Codes.com, RepairPal)
A short to power or to ground in either CAN bus wire (OBD-Codes.com)
A weak or dead battery (RepairPal)
A problem with the ABS module circuit, such as damaged or corroded wiring and connectors (OBD-Codes.com, RepairPal)
A faulty ABS module or module software; OBD-Codes.com says a bad module is rare, while RepairPal lists a faulty ABS module as a common cause

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Check for other codes first. RepairPal says any other codes should be addressed before U0121, and several communication codes at once point to the CAN network.
Make sure the battery is fully charged and its connections are clean and tight, then clear the code and see if it returns (RepairPal).
Try to read codes from the ABS module with a scan tool. OBD-Codes.com says if you can, the code is intermittent or stored; if you can't, the fault is active now.
Check the fuses that power the ABS module and make sure its ground points are clean and tight (OBD-Codes.com).
Disconnect the negative battery cable, then inspect the ABS module connector and wiring for rubbing, bare wires, burn marks, melted plastic or green corrosion on the terminals (OBD-Codes.com).
Check for technical service bulletins (TSBs) for your vehicle that mention U0121 (OBD-Codes.com, RepairPal).

Typical fixes

Charge or replace a weak battery (RepairPal)
Replace a blown ABS module fuse or clean and tighten a bad ground (OBD-Codes.com)
Clean corroded ABS module connector terminals and repair damaged wiring (OBD-Codes.com, RepairPal)
Repair an open or shorted CAN bus wire (OBD-Codes.com, RepairPal)
Reprogram the ABS module software, which RepairPal says can sometimes avoid a replacement
Replace the ABS module only after power, ground and network checks pass; OBD-Codes.com says most must be programmed or calibrated to the vehicle

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The ABS, traction control or stability control light stays on, since those safety systems may not be working
A scan tool can't communicate with the ABS module at all
Several modules are reporting lost communication, which calls for network testing with a multimeter or breakout box (RepairPal)
The ABS module needs reprogramming or replacement, which needs professional scan tools (OBD-Codes.com, RepairPal)

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is U0121 serious?

RepairPal rates it Moderate. It usually turns on the ABS light, and on some cars the traction and stability control lights (OBD-Codes.com), so those features may not be working. Get it diagnosed soon and drive with extra care until then.

Can a bad battery cause U0121?

Yes. RepairPal says U0121 can result from a dead battery and that the battery should be checked, charged or replaced before other testing. On many vehicles the code only sets when battery voltage is between 9 and 16 volts.

Do I need a new ABS module for U0121?

Not necessarily. OBD-Codes.com says loss of power or ground is the most common cause and a faulty module is rare. RepairPal says the module can sometimes be reprogrammed instead of replaced, and OBD-Codes.com says most replacement modules must be programmed to the vehicle.

How much does it cost to fix U0121?

Neither source gives a repair cost. It can range from replacing a fuse or cleaning a ground to reprogramming or replacing the ABS module, so get a diagnosis or a written estimate first.

Sources

Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.