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U0121 Severity
Once the vehicle's onboard system no longer detects the issue, most codes will disappear on their own. If your vehicle still has this code, however, it should be properly diagnosed to avoid a more serious problem.
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Moderate severity codes might not threaten the vehicle's safety or functionality immediately, but you should still get it diagnosed within a reasonable time frame to keep your car running well.
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Code U0121 stands for Lost Communication with Anti-lock Brake System Module.
Anti-lock brakes are required on all modern vehicles. The antilock braking system (ABS) prevents wheel lockup by modulating the brakes in an emergency situation. Basically, the ABS system does what the driver used to do in the old days - pump the brakes during a panic stop.
Anti-lock Brake System
To accomplish this, the ABS system relies on complex hydraulics and electronics.
The ABS module communicates with other onboard computers (referred to as modules) over the controller area network (CAN) bus. The bus is composed of two lines: CAN High and CAN Low. Modules communicate over the CAN High bus at a rate of 500k bits/second, whereas communication over the CAN Low bus is limited to 125k bits/second. At the ends of the bus, there are two terminating resistors.
Code U0121 indicates the ABS module is not receiving or transmitting messages on the CAN bus.
Get it diagnosed by a professional
Code U0121 is typically caused by one of the following:
Sometimes U0121 can pop up intermittently, or it can result from a dead battery. This is especially true if the code is a history code and not current. Clear the code and see if it returns. If it does, the next step is to perform a visual inspection. A trained eye can check for issues such as broken wires and loose connections. If a problem is found, the issue should be repaired and the code cleared. If nothing is discovered, check for technical service bulletins (TSBs). TSBs are recommended diagnostic and repair procedures put out by the vehicle manufacturer. Finding a related TSB can greatly reduce diagnostic time.
The ABS module requires a proper voltage level to operate. Before proceeding, check the vehicle battery. Charge or replace the battery as needed and clear the code.
Additional diagnostic trouble codes (DTCs) may indicate problems elsewhere that are affecting ABS module operation. For example, multiple communication DTCs may indicate a problem with the CAN network. Any additional DTCs should be addressed prior to diagnosing U0121.
In the case where multiple communication DTCs are stored, diagnosis will shift to the CAN bus. Like any other electrical circuit, the bus can be checked for problems such as opens and shorts. Bus testing usually starts are pins 6 and 14 of the data link connector. These are the CAN High and CAN Low pins. If a problem is detected, further testing can be performed at additional locations throughout the CAN network. Testing is typically done with a digital multimeter (DMM) and/or breakout box.
If U0121 is the only DTC stored, the ABS module itself should be checked. In most cases, a professional will start by trying to communicate with the module using a diagnostic scan tool. The tool connects directing to the vehicle’s diagnostic port. Once connected, it can communicate with all modules on the network.
An ABS module that does not respond to the scan tool will need to be diagnosed. Before condemning the ABS module, its circuit should be checked with a DMM. Like any other electrical device, the ABS module must have proper power and ground.
If everything up to this point checks out OK, the ABS module is likely the problem. Before replacing the ABS module, its software should be checked. Sometimes, the ABS module can be reprogrammed instead of replaced. If software isn’t the issue, the ABS module is faulty and requires replacement.
All the 'U' codes are network communication codes. Codes U0100 to U0300 are lost communication with XX module codes.
On many vehicles, battery voltage must be between 9 – 16 volts for code U0121 to set.