U0121 on Genesis: How to Diagnose & Fix
Lost Communication with ABS Module
1 Genesis model · 1 documented issue · 1 critical · $400-$2,500 repair
U0121 on Genesis by model: symptoms, diagnosis and fix
Multiple 2023 G90 owners report sudden 'dashboard lit up like a Christmas tree' events: every warning light illuminates, a 'STOP ENGINE / check brake system' message appears, and the electronic parking brake refuses to release, stranding the vehicle. One documented dealer diagnosis was a shorted main sensor, but the fault is intermittent and recurred one week after repair. Separate 2023 G90 electrical complaints (dead battery, door handles not deploying) suggest a broader pattern of low-voltage / sensor faults on the new RS4 generation.
Common Symptoms
- all dashboard warning lights illuminate simultaneously
- STOP ENGINE / check brake system message
- electronic parking brake malfunction
- vehicle may be immobilized
- intermittent re-occurrence after sensor replacement
- associated parasitic battery drain
- associated false TPMS warnings
How to Fix
Document every occurrence with date, mileage, and a photo of the cluster. Demand a parasitic-draw test and scan of all CAN modules. If the fault recurs after sensor replacement (commonly the body-control or yaw module), escalate to Genesis case management — multiple owners have qualified for lemon-law buyback once events exceed 30 days down or 3+ repair attempts.
Sources for this issue (1)
U0121 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does U0121 mean on a Genesis?
U0121 stands for "Lost Communication with ABS Module." U0121 is a network communication code meaning a control module on the vehicle's data bus has lost communication with the ABS (anti-lock brake system) module. Modern cars share information over a CAN bus, and when other modules stop receiving expected messages from the ABS unit, this code sets. Because the ABS module also handles traction control and stability control on most vehicles, you may see multiple warning lights and lost features. It points to a communication or power problem with the ABS module rather than a worn brake part. On Genesis it is documented across 1 model: G90.
What are the most common causes of U0121?
The most common causes of U0121 are: Loss of power or ground to the ABS module, Blown fuse feeding the ABS controller, Damaged, corroded, or loose ABS module connector, Open, shorted, or chafed CAN bus wiring, Internal failure of the ABS control module, Corrosion or water intrusion in connectors/harness, Faulty gateway module disrupting bus communication. The specific cause varies by vehicle.
How much does it cost to fix U0121 on a Genesis?
Published repair estimates on Genesis range from $400 to $2,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for U0121 on a different make?
View U0121 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.