U0121

Code

Generic U — Network/User

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

Brand: Generic

Type: U — Network/User

Views: UK: 55 EN: 359 RU: 226

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Connect a scan tool and confirm U0121 and any additional communication codes
Verify battery voltage is within specification (key on and cranking)
Inspect ABS module fuse(s) and related relays for continuity
Visually inspect ABS module connector and wiring for corrosion, pin damage, or water intrusion
Measure +12V power and ground at the ABS module connector
Measure CAN bus voltages: CAN_H/CAN_L open-circuit and with key on

CAN bus recessive voltage: ~2.5 V on CAN_H and CAN_L (key on, no dominant bits)
CAN dominant state: CAN_H ~3.5–4.0 V, CAN_L ~1.0–1.5 V during traffic
Termination resistance: ~60 ohms between CAN_H and CAN_L (two 120 ohm resistors in parallel)
ABS module message interval: typically tens to hundreds of ms (varies by manufacturer)
No valid ABS message ID or sporadic/erratic message timing when U0121 present

Use a scan tool: read and record U0121 and any other DTCs, attempt to access ABS module data and note 'no response' or timeouts.
Check vehicle battery voltage and charging system; low voltage can cause communication faults.
Inspect ABS module fuse(s) and relays; replace or restore if blown/faulty and re-test communication.
Visually inspect ABS connector and wiring harness for damage, corrosion, water entry, or bent pins; repair as needed.
With ignition on, backprobe ABS connector: verify +12V supply and good ground. Repair power/ground faults before further testing.
Measure CAN_H and CAN_L voltages at the ABS connector with key on. Compare to expected values (see signal_params).
Measure resistance between CAN_H and CAN_L with ignition on (expect ~60 ohms). If open or out of range, inspect wiring and terminators.
If voltages/termination look good, use an oscilloscope to view CAN waveforms for noise, missing messages, or dominant bus conditions.
Isolate fault by disconnecting other modules or harness branches (one at a time) to find source of bus short or bus-off device.
If wiring and network devices test OK and ABS module still does not communicate, consider replacing or bench-testing ABS control module per manufacturer procedures.
After repairs, clear codes and perform road test and network scan to confirm communication is restored and codes do not return.