U0121
Lost Communication With Anti-Lock Brake System (ABS) Control Module A
Communication / network
Generic OBD-II
High
What it means
U0121 means a control module has stopped receiving messages from the ABS control module over the vehicle's data network. The engine still runs and normal braking works, but ABS, traction control, and stability control usually shut off, with their warning lights on. The cause is most often wiring, a connector, or a power/ground fault at the ABS module rather than the module itself.
High
Can I keep driving? The engine runs and normal brakes work, but ABS, traction, and stability control are likely disabled, so drive cautiously and get it fixed before bad weather.
Common symptoms
What you'll notice
- ABS, traction-control, and stability-control warning lights on
- ABS, traction, and stability features disabled (normal braking still works)
- Check-engine light or other network-related codes may also set
- Speedometer or related readouts may act up on some vehicles
Common causes
Most likely first
- Damaged, corroded, or loose wiring or connector at the ABS module — the most common causeLikely
- Open or short in the CAN/data-bus wiring between modulesCommon
- Loss of power or ground to the ABS control module (check its fuses)Common
- Faulty ABS control moduleLess often
- Rarely, another failed module dragging down the data busLess often
How to diagnose & fix
Work through it in order
- Scan all modules and note any companion U-codes; multiple lost-communication codes point to a shared bus or power/ground fault.
- Check the ABS module's power and ground and its fuses first.
- Inspect the module's connector and the CAN bus wiring for damage, corrosion, or opens, especially near harness flex points.
- Measure the data bus for correct resistance and signal integrity.
- Repair the wiring or power/ground fault; replace the ABS module only after wiring and power are confirmed good, and program it if required.
FAQ
Quick answers
Is it safe to drive with U0121?+
Base brakes still work, but ABS, traction, and stability control are likely offline, so stopping in the wet or on ice is riskier. Drive gently and get it repaired soon.
Can I just clear U0121?+
If it's a one-time glitch it may stay gone, but a wiring, power, or module fault brings it right back. Fix the connection or bus problem rather than just clearing.
Related codes
Often seen with U0121
Read & clear U0121 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX