What Does the Engine Code U0121 Mean?

If your car is displaying the engine code U0121, it means there’s an issue with the anti-lock brake system.
Written by Kaitlin May
Reviewed by Melanie Reiff
When you see the engine code U0121, it means an electrical issue has caused problems with your anti-lock brake system (ABS). The ABS steps in to regulate the brakes in the event of an emergency, so keeping the system well-oiled is key to driving safely—especially in icy winter conditions.
Back in the old days, people had to manually pump their brakes to stop abruptly during a dire situation. Your automated anti-lock brake system does that safely for you. 
It takes a complex set of functions within your vehicle to keep the ABS running, but understanding diagnostic trouble codes (DTCs) can help you identify the source of any problems and address it efficiently. Keeping your car protected is easy with help from the #1-rated
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What does the engine code U0121 mean?

Definition: Anti-Lock Brake System Control Module has lost communication with the CAN bus.
Thanks to modern technology, the ABS module helps keep us safe on the road. It’s composed of intricate electrical systems that communicate with each other so they can operate properly. The controller area network (CAN) bus is the channel where the ABS module sends and receives messages to onboard computers (modules).
The communication system requires two lines: CAN High and CAN Low. Both lines carry an idle voltage and a voltage differential when messages are being exchanged between modules. 
The bus only picks up on the programmed voltage levels and doesn’t account for other noise or electrical fields. The engine code U0121 is triggered when a message from the ABS hasn’t been received.

How much will it cost to fix?

Most maintenance repair shops will charge between $75 and $150 to repair the ABS.

What can cause the U0121 engine code?

Several components of the ABS could be the culprit behind the system’s drop in communication, including:
  • Poor electrical connection in the ABS control module
  • Shorted circuit in the ABS control module
  • Damaged or faulty ABS control module

Common symptoms of the U0121 engine code

As with most engine-related problems, your
check engine light
or other warning lights will illuminate with the U0121 engine code.
You may also experience some drivability issues related to the ABS, like hydroplaning or loss of traction when braking heavily in traffic. 

How serious is the U0121 engine code?

When there’s a split-second decision like swerving out of the way of another vehicle, tree, or obstacle in treacherous driving conditions, a faulty ABS can be the difference between making it home safely and winding up in a serious accident. 
The last thing you want to experience is finding out your ABS is faulty in the event of an accident. Getting your ABS evaluated immediately is essential to maintaining your safety and protecting your car.

Can I fix the U0121 engine code myself?

You can perform a preliminary inspection to diagnose the engine code U0121 on your own before bringing it to a professional to verify the issue.
Since a dead battery can trigger the code, try clearing the code and see if it reappears. Make sure your battery is fully charged, and either recharge or replace it as needed. You can assess the area for any broken circuits or loose wires on your own, but it’s best to get the help of a trained eye to identify the problem and rectify it with the right tools.

How to repair the U0121 engine code

An experienced technician will perform a series of diagnostic scans to rule out system failure sources before replacing the ABS while following these steps:
  • Address additional DTCs that may be triggered
  • Deploy a ​​digital multimeter (DMM) to test the CAN bus
  • Use a scan tool to attempt communication with the ABS
  • Check ABS circuits with a DMM
  • Attempt to reprogram the ABS to verify if there’s a software issue
  • Replace the ABS 

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