U0121 fault code: lost communication with the ABS unit
Plain-language definition
Lost Communication With Anti-Lock Brake System (ABS) Control Module A
What does it mean?
Using information from the wheel speed sensors, the ABS unit stops the wheels from locking under hard braking; on many cars, stability control (ESP) and traction control are also run from the same unit. Wheel speed information is also shared over the data network with modules such as the engine computer, transmission, instrument cluster and electric power steering.
U0121 means the module that stored the code hasn't received the messages it expects from the ABS unit for a set time. The record may be left over from a brief dropout when the battery was weak or the voltage dropped. If the code keeps coming back, the problem is most often in the ABS unit's power supply, its connector or its data network connection; since the ABS unit sits in the engine bay on most cars, moisture can also get into its connector.
You may notice the ABS, ESP and brake warning lights coming on, the speedometer not working or jumping around, erratic gear shifts and cruise control not working. Which symptoms appear depends on which modules get information from the ABS unit.
How serious is it?
Although it's rated Warning, it concerns a brake safety system: when communication with the ABS unit is lost, ABS and stability control (ESP) may not work. That's why you should see a workshop.
This code concerns a safety system: see a workshop before clearing it.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- A problem with the ABS unit's supply fuse or ground connection
- Moisture, corrosion or a loose connection at the ABS unit's connector
- Low battery voltage, or a temporary record left behind during a voltage drop
- Broken wiring, chafing or a short circuit in the data network (CAN) wiring
- A fault in the ABS unit itself
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- GM
- Lost Communication With Electronic Brake Control Module Low Voltage; Lost Communication With Electronic Brake Control Module Invalid Data; Lost Communication With Electronic Brake Control Module Message Counter Incorrect; Lost Communication With Electronic Brake Control Module Bus Signal Checksum Error
- Honda
- FCAN Malfunction (TCS-PCM)
Related codes
How does it appear in the 2dakika report?
Lost Communication With Anti-Lock Brake System (ABS) Control Module A
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.