The most common causes are: Damaged wiring to the ABS module; Loss of power or ground at the module; Corroded connector; Blown fuse; CAN bus wiring fault.

How much does it cost to fix U0121?

Typical repairs range from $5–$30 for replace fuse up to $700–$2,000 for replace abs module (including programming). Costs vary by vehicle and region.

U0121network

U0121 – Lost Communication With Anti-Lock Brake System (ABS) Control Module

What does U0121 mean?

A module has lost communication with the anti-lock brake system (ABS) control module. Because ABS data is shared with traction control, stability control, and sometimes the transmission and engine modules, losing it affects several systems at once.

What causes U0121?

Listed from most to least common:

1Damaged wiring to the ABS module
2Loss of power or ground at the module
3Corroded connector
4Blown fuse
5CAN bus wiring fault
6Failed ABS module
7Low system voltage

Code breakdown

SystemU — Network (User)

TypeGeneric (standardized SAE / OBD-II code — same meaning across most manufacturers)

Common symptoms

ABS, traction control, and stability control warning lights on
Those systems disabled
Scan tool cannot communicate with the ABS module
Possible speedometer issues on some vehicles
Normal braking retained

How to diagnose

1Check battery and charging system voltage first.
2Check ABS module fuses, including any high-current fuses.
3Verify power and ground at the ABS module connector.
4Inspect wiring near the module, which is often mounted in an exposed underhood location.
5Check for corrosion and moisture at the connector.
6Test CAN bus resistance and check for other modules reporting U-codes.

ABS, traction control, and stability control are typically disabled. Base braking still works, but the safety systems that assist in emergency stops may not. Drive with extra caution until repaired.

Common fixes and estimated costs

Repair	Estimated cost
Replace fuse	$5–$30
Clean/repair connector	$100–$400
Repair wiring	$200–$800
Replace ABS module (including programming)	$700–$2,000

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

ABS modules are frequently mounted where they are exposed to heat and moisture, so connector corrosion is a leading cause. Because ABS affects braking safety, have this diagnosed promptly rather than driving on it indefinitely.

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This information is provided for general reference only and may vary by manufacturer, model, and year. It is not a substitute for professional diagnosis or repair.