U0121

Code

LAND ROVER U — Network/User

Lost communication with the ABS control module

Brand: LAND ROVER

Type: U — Network/User

Views: UK: 32 EN: 171 RU: 102

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Page language: EN

Causes

ABS control module not powered or no ground
Faulty ABS module (internal failure)
Open, short or high resistance in CAN bus wiring (CAN_H / CAN_L)
Missing or incorrect termination resistor on CAN bus
Corroded / loose connectors at ABS module or other network nodes
Blown fuse supplying ABS module or related control module

Symptoms

ABS warning lamp ON (ABS/ESP/Traction control lights)
Loss of ABS/traction control function; ABS not active during braking
Possible vehicle stability/traction warning messages
Cruise control or other systems relying on ABS wheel speed may be disabled
No response from ABS module in a diagnostic scanner (module not present)

What to check

Scan with a capable diagnostic tool: confirm U0121, read freeze frame and related U-codes
Check for other module communication faults (U-codes) — indicates wider CAN problem
Verify battery voltage (engine off and cranking) and charging system health
Visually inspect ABS module connector and wiring for corrosion, pin damage, water ingress
Check ABS power/ignition fuses and relays
Measure power and ground at ABS module connector (ignition ON)

Signal parameters

CAN bus recessive common‑mode ≈ 2.5 V (both CAN_H and CAN_L near 2.5 V)
CAN bus dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Termination resistance ≈ 60 Ω between CAN_H and CAN_L with ignition ON (two 120 Ω in parallel)
Typical CAN data rate for ABS: 250 kb/s or 500 kb/s depending on vehicle (verify with OEM spec)
Module supply: nominal 12 V (approx. 11–14 V while running); ground resistance should be very low (

Diagnostic algorithm

Capture and record all communication U-codes and freeze frame data with a full OEM-capable scanner.
Inspect ABS module area for corrosion, water, impact damage and check connector for bent pins or contamination.
Verify vehicle battery voltage and charging system; ensure stable 12 V supply during testing.
Check fuses and relays supplying the ABS module; replace if blown and re-test.
With ignition ON, backprobe ABS module connector to confirm proper battery/ignition power and ground continuity.
Measure termination resistance across CAN_H and CAN_L (~60 Ω). If open or incorrect, trace to find missing/failed termination.
Use a 2‑channel oscilloscope or high-speed CAN tool to view CAN_H and CAN_L signals while cranking/running — look for noise, bus-off conditions, or missing frames.
If bus activity present but ABS module not responding, try resetting/clearing codes then power cycle; if persists, attempt reprogram/reflash per OEM procedure.
Trace and repair any wiring harness faults (shorts to battery/ground, opens, high resistance) between ABS module and nearest gateway/module.
If wiring and power/ground are good and problem persists, consider bench testing or replacing ABS control module and program/configure to vehicle.
After any repair, clear codes and verify system operation and communication longevity; road test to confirm ABS/ESP function.

Likely causes

Corroded connector at ABS module (water intrusion common)
Damaged CAN wiring harness (chafing near suspension/wheels)
Blown ABS power fuse or bad power/ground circuit
ABS module has gone bus‑off or failed after impact or water damage
Incorrect programming or module not configured to vehicle

Fault status

⚠️ Status

Lost communication with ABS control module — ABS module not responding on vehicle CAN network. Verify power/ground, fuses, CAN bus integrity, connectors, and module programming before module replacement.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 3.0 hours

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U0121 — Lost communication with the ABS control module