U0121 means Lost Communication With Anti-Lock Brake System (ABS) Control Module. A module on the vehicle network has stopped hearing from the ABS control module. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What U0121 means
Modern vehicles have many control modules that share information over a network, usually called the CAN bus. Codes starting with U mean a module has stopped hearing another one, and voltage and processor codes mean a module is not getting clean power or has an internal fault. Low battery voltage, poor grounds, blown fuses and damaged wiring cause many of these codes, so test the battery and charging system first.
U0121 symptoms
- ABS, traction and stability lights on
- Speedometer problems on vehicles that use ABS wheel speed
- Erratic shifting on some vehicles
Common causes of U0121
- ABS module without power or ground (fuse or corroded ground)
- Damaged network wiring or connector at the ABS module
- Low battery voltage
- Failed ABS module
How to diagnose U0121
- Check ABS fuses, power and ground
- Inspect the module connector for corrosion, since it often sits near a wheel well
- Check whether a scan tool can communicate with the ABS module
Can you drive with U0121?
Carefully: normal brakes work but ABS and stability control are off.
U0121 tips
ABS modules often sit low in the engine bay near a wheel well, where water and salt corrode the connector. Clean and seal the connector before replacing the module.
Will U0121 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing U0121 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| U0100 | Lost Communication With ECM/PCM A |
| U0073 | Control Module Communication Bus A Off |
| P0500 | Vehicle Speed Sensor A Malfunction |
Guides that help
When it is more than a sensor
Voltage and network codes come from wiring, the battery, the charging system or a module, not from engine or transmission wear. If a lost-communication code has left the transmission in limp mode, rule out the module and wiring before deciding the unit has failed, and see our transmissions only if testing confirms internal damage.
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