U0121 Code: Lost Communication with ABS — Causes & Fix
U0121 means lost communication with the anti-lock brake system (ABS) control module — the network can no longer talk to the ABS/stability computer.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $50 – $900
- Time to fix
- 1–4 hours
What it means
Causes include ABS module power/ground loss, CAN wiring faults, or a failed ABS module. U0121 is a generic OBD-II network/communication ("U") code defined in the SAE J2012 standard, so it applies to virtually any modern OBD-II vehicle regardless of make. These faults usually point to the vehicle's data bus (often CAN) or a module that has dropped off the network. Fix the wiring/module fault, then clear the code with a scan tool.
Symptoms
- ABS, traction, and stability lights on
- Speedometer may be affected
- Those safety systems disabled
How to fix it
- 1
Check ABS power and wiring
1. Verify ABS module power and grounds. 2. Inspect the CAN wiring to the ABS module.
- 2
Diagnose the module
1. Scan whether the ABS module responds. 2. Repair wiring or address the module, then clear the code.
Frequently asked questions
Is it safe to drive with U0121?
Your normal brakes still work, but ABS/traction/stability control are likely disabled. Drive gently and get it fixed — those safety systems are off.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.