U0120

Code

LAND ROVER U — Network/User

Lost communication with the starter relay control module generator

Brand: LAND ROVER

Type: U — Network/User

Views: UK: 25 EN: 120 RU: 79

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Page language: EN

Causes

Blown fuse or loss of power to the control module
Poor or open ground connection at the module
Damaged, corroded or disconnected wiring or connectors
Faulty starter/relay/generator control module
CAN bus wiring short, open or high resistance
Missing or corrupted module software (firmware)

Symptoms

No-crank or intermittent crank condition
Starter or generator (alternator) control functions inoperative
Relevant warning lamp(s) on the dash (starter/charging/warning lights)
Multiple network-related DTCs or loss of other related modules' functions
Intermittent electrical faults that may clear when codes are cleared

What to check

Connect a diagnostic scanner and read stored and pending U-codes and freeze frame data
Verify battery voltage is stable (12.4–12.8 V at rest; ~13.5–14.8 V when running)
Inspect fuses and relays for the starter/alternator control circuits
Visually inspect module connectors for corrosion, bent pins or water intrusion
Check for other active U-codes that indicate network-wide issues
Measure power and ground at the control module connector

Signal parameters

Battery/supply voltage to module: ~12–14.8 V (with engine running ~13.5–14.8 V)
Ground: ~0 V, low resistance to chassis ground (< 1 Ω expected at connector)
CAN bus idle: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V
CAN bus dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
Bus termination resistance: ≈ 60 Ω between CAN_H and CAN_L (measured with key off)
Expected message presence: module-specific CAN message ID(s) at the network data rate (verify with scope/scan tool)

Diagnostic algorithm

Record all codes and freeze-frame data. Note when the code sets and any related codes.
Verify battery voltage and condition. Recharge or replace battery if below recommended levels.
Clear codes and attempt to recreate the fault to verify persistence.
Inspect fuses/relays for the starter/alternator control module circuit and replace if blown.
Visually inspect and reseat the control module connector(s). Look for corrosion, bent pins, water ingress or pushed-out terminals.
Measure supply voltage and ground at the module connector with key on and cranking. Repair any open or low supply/ground circuits.
Measure CAN_H and CAN_L at the module connector and at a known-good node. Check for correct idle voltages, dominant voltages and proper termination resistance.
Perform a wiggle test or backprobe while monitoring CAN traffic to attempt to reproduce the communication loss and locate intermittent wiring faults.
If CAN is inactive at the module but active elsewhere, trace wiring between module and bus, repair short/open as required.
If wiring, power and ground are confirmed good but no communication, verify module software level and reflash/update if recommended by manufacturer.
As a last step, swap in a known-good module (if available) or replace the module. After repair, clear codes and verify proper operation and message presence on the CAN bus.
Document repairs and re-scan to ensure no related U-codes remain and the vehicle functions as intended.

Likely causes

Connector corrosion or open power/ground at the control module
CAN bus wiring fault (open, short to ground or +12V)
Blown fuse feeding the module
Failed control module

Fault status

⚠️ Status

Lost communication with starter/relay/generator control module — module not responding on the vehicle CAN network.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0 - 3.0 hours

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U0120 — Lost communication with the starter relay control module generator