What Does U0121 Mean?
The control module has lost communication with the Anti-Lock Brake System (ABS) control module over the vehicle's network. This typically results in loss of ABS, traction control, and stability control functions.
Safety Warning
ABS, traction control, and stability control systems will be disabled. Vehicle retains basic braking but lacks anti-lock and stability assistance, significantly increasing accident risk in emergency maneuvers or slippery conditions.
Common Causes
35%
Faulty ABS control module with internal communication circuit or processor failure
30%
CAN bus wiring harness damage including opens, shorts, or damaged insulation
20%
Corroded or damaged ABS module connector terminals
10%
Power supply failure to ABS module (blown fuse, open circuit, failed relay)
5%
Body control module or gateway module communication failure
Diagnostic Steps
1
Step 1: Check ABS fuse and relay in power distribution center; verify fuse integrity and relay operation
2
Step 2: Inspect ABS module connector for water intrusion, corrosion, or damaged pins; clean and secure connection
3
Step 3: Measure voltage at ABS module power and ground circuits with key on; verify proper battery voltage present
4
Step 4: Check CAN bus voltage at ABS module connector (CAN high should be ~2.5-3.5V, CAN low ~1.5-2.5V with ignition on)
5
Step 5: Measure CAN bus resistance between high and low circuits with modules disconnected (should be approximately 60 ohms); check for short to ground or power
Estimated Repair Cost
$150 - $1,800
Parts + labor, varies by vehicle and location
The control module has lost communication with the Anti-Lock Brake System (ABS) control module over the vehicle's network. This typically results in loss of ABS, traction control, and stability control functions.
The most common cause of U0121 (Lost Communication With Anti-Lock Brake System (ABS) Control Module) is: Faulty ABS control module with internal communication circuit or processor failure
Typical repair costs for U0121 range from $150 to $1,800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
ABS, traction control, and stability control systems will be disabled. Vehicle retains basic braking but lacks anti-lock and stability assistance, significantly increasing accident risk in emergency maneuvers or slippery conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0121 to identify the root cause.
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Quick Info
Category
Network
System
Anti-Lock Brake System / Communication Network
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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