What U0121 means
U means network (communication between modules). The 0 after it marks a generic code defined by SAE rather than by the manufacturer, and the U01xx block is reserved for lost-communication faults with specific modules.
Modern vehicles connect their control modules on a shared data network, most often CAN bus. Each module expects regular messages from the others. A "lost communication" code is set by a module that stopped receiving expected messages from the named module for longer than an allowed time.
The module named here is the anti-lock brake system control module. What you notice depends on what that module does: aBS and stability control warnings; normal braking usually remains but anti-lock function may not.
What “Lost Communication” means here
A module on the network expected periodic messages from the anti-lock brake system control module and did not receive them for longer than allowed. The code is stored in the module that noticed the silence, not necessarily in the one that went quiet, and the named module may itself be unpowered, faulty, or cut off by wiring.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany U0121:
- Warning lights for the system the missing module controls
- Features of that system not working
- Several U-codes stored at once in different modules
Where to start diagnosing U0121
- Check battery voltage and the charging system first; low voltage causes many communication faults.
- Check whether the named module is powered: its fuses, power feeds and grounds.
- Scan all modules: if one module is silent to every other module, focus on it; if many modules are missing, suspect the network wiring.
- Inspect network wiring and connectors near the named module; on CAN, the two termination resistors should measure about 60 Ω together across the bus pins with the battery disconnected.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to U0121. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Control module voltage (PID 0x42) | Low system voltage can cause communication faults. |
| Module list | A full network scan showing which modules respond. |
Related codes
Codes from the same vehicle network communication family. If several are stored together, the shared network wiring, power supply or ground is the place to start.
| Code | Definition |
|---|---|
| U0114 | Lost Communication With Four-Wheel Drive Clutch Control Module |
| U0115 | Lost Communication With ECM/PCM "B" |
| U0122 | Lost Communication With Vehicle Dynamics Control Module |
| U0123 | Lost Communication With Yaw Rate Sensor Module |
| U0124 | Lost Communication With Lateral Acceleration Sensor Module |
| U0125 | Lost Communication With Multi-axis Acceleration Sensor Module |
| U0126 | Lost Communication With Steering Angle Sensor Module |
| U0127 | Lost Communication With Tire Pressure Monitor Module |
Further reading
Frequently Asked Questions
What does U0121 mean?
U0121 is the generic SAE J2012 code for "Lost Communication With Anti-Lock Brake System (ABS) Control Module". In plain terms, it means another module on the vehicle network stopped hearing from the anti-lock brake system control module.
Can I keep driving with U0121?
ABS and stability control warnings; normal braking usually remains but anti-lock function may not. Because this module is involved in braking, steering, stability or propulsion, have it checked before driving further than you need to.
What should I check first for U0121?
Check battery voltage and the charging system first; low voltage causes many communication faults.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.