Lost Communication With Anti-Lock Brake System Module
The U0121 code means the vehicle's onboard diagnostic system has detected: Lost Communication With Anti-Lock Brake System Module. This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- hard
- Estimated cost
- $200 - $1200 (parts and labor)
Network codes mean modules are not communicating. You need a scanner that reads all systems to find the problem.
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Common Symptoms
- ABS warning light illuminated
- Check Engine Light may be on
- ABS system inoperative
- Traction control and stability control disabled
- Normal braking still works
Probable Causes (Ranked by Likelihood)
- Failed ABS module Very High
- CAN bus wiring fault to ABS module High
- Corroded ABS module connector Moderate
- ABS module power supply loss Moderate
Step-by-Step Diagnostic Procedure
-
Check ABS module power and ground circuits
-
Inspect ABS module connector for corrosion -- common due to low mounting position
-
Check CAN bus wiring to ABS module
-
Verify ABS pump motor operates when commanded
-
Replace ABS module if communication cannot be restored
Common Fixes by Vehicle
What techs usually find when diagnosing U0121 on specific vehicles — tap your vehicle for the fix and the exact part:
2007-2018 Chevrolet Silverado/GMC Sierra easy DIY
Lost communication with ABS module. The EBCM (Electronic Brake Control Module) on GM trucks is mounted on the ABS hydraulic unit in the engine bay. The connector corrodes from road salt spray. Unplug the EBCM connector, inspect for green corrosion on pins, clean with contact cleaner, and apply dielectric grease. If the pins are damaged, the connector pigtail can be replaced separately.
Labor: 30 min - 1 hour2008-2020 Ford F-150/Explorer/Escape moderate DIY
Ford ABS module communication loss is often caused by a failing ABS module power supply. Check the ABS fuse in the battery junction box and the ABS module ground. The ABS module ground is on the frame near the module -- clean the ground connection. If power and grounds are good, the ABS module itself may have an internal CAN bus transceiver failure, requiring replacement.
Labor: 30 min - 2 hours2007-2017 Chevrolet Tahoe/Suburban/Yukon moderate DIY
Same EBCM connector corrosion issue as the trucks. On full-size SUVs, also check for CAN bus wiring damage where the harness crosses the steering intermediate shaft. The shaft can rub through the wiring loom and damage the CAN-H or CAN-L wire to the EBCM. Look for abraded wiring insulation and repair with solder and heat shrink.
Labor: 30 min - 2 hoursFrequently Asked Questions
Can I drive with code U0121?
Drive short distances only. Lost communication codes indicate a module has stopped communicating on the vehicle network. The affected system may not function correctly.
What causes U0121?
The most common cause is a failed module or a wiring issue in the CAN bus network. Failed ABS module is the most likely cause. Power supply, ground, and connector issues should be checked before replacing any module.
Is U0121 expensive to fix?
Repair costs range from $200 - $1200 (parts and labor). Wiring repairs are on the lower end, while module replacement is on the higher end. The module may also need to be programmed after replacement.
Sources
This page is built from documented references. Verify against your own service info before repair work.
U0121 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
See All Codes & Fixes for Your Vehicle
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