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

What does U0121 mean?

The ABS module shares wheel speed and brake data with other modules over the network. When they stop hearing from it, they store U0121, and ABS related warning lights usually come on.

Modern cars connect their control modules over a Controller Area Network (CAN). When one module stops hearing messages it expects from another, it stores a lost communication U code. The module named in the code may be faulty, but power, ground, battery and network wiring faults can set the same code. Manufacturer bulletins advise confirming power, ground, battery voltage and network resistance before replacing a module.

Common symptoms

• Check engine light on
• ABS warning light on
• Traction control or stability control light on, depending on the car

Common causes

The causes below are the ones commonly seen with U0121. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• Loss of power or ground to the ABS module
• Opens or shorts in the CAN bus wiring
• Low battery voltage
• A faulty ABS module (rare)

What to check with a scanner

• Read codes from every module the app can reach, not only the engine. Note which modules report lost communication with the ABS module.
• Check battery voltage in Live data with the engine off and running. Low battery voltage is a known trigger for communication codes.
• Diagnose any battery, charging or power supply codes first, then the communication code.
• Clear the codes once and see whether they return. A code that only comes back now and then often points to a loose connection or ground.

How to read and clear U0121 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm U0121 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up U0121 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for U0121.

When to see a mechanic

Get professional help if any of these apply:

• The car will not start, stalls or the transmission stays in one gear
• The code returns after the battery and fuses have been checked
• Wiring needs to be tested with a meter at the module connector
• A control module may need to be replaced, which usually requires programming to the vehicle
• The brake warning light (not only ABS) is on

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Related trouble codes

See all OBD2 trouble code guides

Related guides

Check engine light on: what to do · How to read an OBD2 code (P, B, C, U) · Stored, pending and permanent codes and freeze frame · OBD2 readiness monitors explained · OBD2 glossary · OBD2 by car brand