U0121 - DATA BUS: ANTI-LOCK BRAKE SYSTEM (ABS) CONTROL MODULE - NO COMMUNICATION

While the vehicle is operating normally, the control modules are connected via the Engine General Local Area Network (GMLAN). Its function is to monitor the output data from all the control modules that are related to the operation of the vehicle. Then, this data is compared to the network of computers that monitor the operating systems in an automated manufacturing environment.

The GMLAN system is like a clearinghouse for all the information exchanged between the vehicle modules. This system also monitors the communication of the modules, which contains information about the signals to be exchanged between them.

The U0121 OBDII diagnostic code is set when communication between the ABS Brake Control Module and the PCM is lost for a period. In most cases, it occurs after a communication interruption of 250 milliseconds (0.25 sec).

• Check Engine light is on.
• The motor does not start.
• Acceleration is limited.
• Fail-safe mode activated.

The reasons for setting the DTC U0121 OBD2 code are:

• There may be cables or connectors in bad condition.
• The ABS Control Module may be defective.
• If there was a recent replacement of a control module not related to this system, communication may have been lost.
• The battery may send excessively low voltages or be in poor condition.

To troubleshoot the U0121 OBDII fault code, consider this:

• First, record all fault codes that may be present. If you find such codes, diagnose them before
• Consult the service manual for the location of the wiring and the ABS Brake Control Module. Check for obvious damage to wirings such as wear, corrosion, or burns. Repair as required.
• Follow the guidelines in the service manual, test the resistance, reference voltage, continuity, and ground signal of the system wiring harness. If you detect an out-of-range value, make the required corrective actions.
• On certain vehicles such as Mazda cars, this code is set if a code reader or scanner is connected during engine operation. In this case, it is recommended to erase the code following the manufacturer's instructions.