U0018

Code

LAND ROVER U — Network/User

Medium speed CAN communication bus (â €) short to dam (+)

Brand: LAND ROVER

Type: U — Network/User

Views: UK: 28 EN: 55 RU: 40

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Page language: EN

Causes

Damaged or pinched CAN wiring harness contacting battery positive or fused power circuit
Corroded or damaged connector pins causing a high-voltage path into the CAN pair
Failed CAN transceiver (inside a module) leaking battery voltage onto the bus
Aftermarket accessory incorrectly tied into vehicle power or CAN wiring
Water intrusion in a module or connector creating conductive path to B+
Previous accident repair with chafed or mis-routed wiring

Symptoms

Multiple related modules on the medium-speed CAN bus report lost communication or throw warning lamps
Functions controlled by medium-speed CAN (e.g., some body/comfort features, door modules, HVAC depending on model) inoperative or intermittent
Stored network communication DTCs in one or more modules
Possible parasitic battery drain when vehicle is off (if short present continuously)
Intermittent operation that may clear when harness movement changes the short

What to check

Read and record all network DTCs from all modules with a capable OEM-level scan tool
Visually inspect CAN wiring, harnesses, and connectors for damage, pinched wires, corrosion, or water ingress (doors, tailgate, fuse box, modules)
With ignition off, measure resistance/continuity between CAN wires and battery positive; a low resistance indicates a short to B+
Measure resistance between each CAN line and chassis ground; confirm no direct short to ground
With ignition ON (engine off), measure voltage on the CAN lines — check for abnormal voltages pulled toward battery voltage
Disconnect modules on the medium-speed CAN network one at a time to see if the fault clears (isolation method)

Signal parameters

Expected (generic CAN) idle/recessive: CAN_H and CAN_L near the same bias (~2.5 V each) when undriven
Expected dominant state: CAN_H higher (about ~3.5 V), CAN_L lower (about ~1.5 V) producing ~2 V differential
Short-to-B+ symptom: one or both CAN lines pulled toward battery voltage (~12 V) or higher than normal bias
Short-to-ground symptom: one or both CAN lines pulled near 0 V
Intermittent short may show normal voltages at rest and abnormal spikes under movement or certain conditions

Diagnostic algorithm

Retrieve and record all U-codes and related network codes from every module; note which modules report bus loss.
Perform a careful visual inspection of medium-speed CAN harness routing (doors, tailgate, fusebox, ECU connectors). Look for chafing, corrosion, water entry, or aftermarket tap-ins.
With ignition off, isolate the medium-speed CAN wiring and measure resistance to battery positive on each CAN conductor. A low resistance indicates a short to B+; if so, trace toward the short by disconnecting connectors/splices sequentially.
If resistance to B+ is high/normal, reconnect and turn key ON (engine off) and measure line voltages. Use a lab scope or CAN analyzer to check for a line stuck high or irregular waveforms.
Disconnect modules one at a time (starting with easily accessed body modules) while monitoring scan tool codes or bus waveform to identify the module or branch that, when disconnected, clears the short or fault.
Inspect/repair any damaged wiring, pins, or corroded connectors found. Replace a module if its transceiver is diagnosed as the source (follow OEM replacement/procurement and reprogramming procedures).
After repairs, clear codes, then perform a full network scan and functional checks of affected systems. Road test and recheck for reappearance of U0018.
If no wiring fault is found, consult OEM wiring diagrams for medium-speed CAN topology and consider module bench-testing or replacement following OEM diagnostic flow.

Likely causes

Short to B+ at a connector or splice on the medium-speed CAN harness
Failed module transceiver applying battery positive to the bus
Local harness damage at body/door/ECU harness routing (doors, tailgate, HVAC)
Corrosion or moisture in a control module connector

Fault status

⚠️ Status

Medium-speed CAN bus short to battery positive detected. Over-voltage on medium-speed CAN wiring prevents normal network communication; related modules may lose communication or become inoperative until the short is removed.

🔴 Repair difficulty: Hard

⏱️ Diagnostic time: 2-6 hours

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U0018 — Medium speed CAN communication bus (â €) short to dam (+)