Code
U1A00
LAND ROVER
U — Network/User
Inter-module communication — missing or invalid network message
Views:
UK: 5
EN: 15
RU: 9
AI status
Completed
Completed
100%
Causes
- Damaged, corroded or loose connectors on the network wiring
- Open or short in CAN/communication bus wiring (short to battery/ground)
- Failed or intermittent control module (sender or receiver)
- Missing or incorrect termination resistor(s) on the bus
- Low battery/unstable vehicle voltage or poor ground
- Software mismatch, corrupted module programming, or required reflash/update
Symptoms
- Communications-related warning lamps (e.g., message bus, generic warning light)
- Loss of function for one or more systems (e.g., ABS, traction, instrument cluster, HVAC)
- Intermittent or permanent inoperative modules or subsystems
- Multiple related U-codes or chained faults present
- Vehicle enters limp/limited functionality mode for affected systems
What to check
- Connect an OEM-calibrated scan tool; read all U-codes and module presence list
- Note which module(s) report the U1A00 and any accompanying codes; record freeze-frame data
- Verify battery state-of-charge and charging system voltage (should be stable ~12.6–14.6 V)
- Visually inspect CAN/communication harness, connectors and grounds for damage or corrosion
- Check connector pin fitment and wiring continuity between modules and junctions
- Measure termination resistor across CAN_H and CAN_L at key junctions (typically ~60 ohms total)
Signal parameters
- Idle (recessive) voltage: both CAN_H and CAN_L approximately 2.5 V (vehicle dependent)
- Dominant state: CAN_H rises and CAN_L falls — differential ~1.5–2.5 V (typical)
- Termination resistance: two 120 ohm resistors in parallel ≈ 60 ohms across CAN_H and CAN_L
- Common bus data rates: 125 kb/s, 250 kb/s, 500 kb/s or higher — verify vehicle-specific baud rate
- Expected presence: specific message IDs at defined frequencies (use OEM message list)
Diagnostic algorithm
- Scan and document all stored codes, freeze-frame and module list. Identify which module logged U1A00 and any related U-codes.
- Check battery voltage and ground integrity; poor supply can cause message loss. Recharge or isolate charging faults first.
- Visually inspect and wiggle-test main communication wiring, connectors at modules and junction boxes for corrosion, looseness, or damage.
- Measure termination across CAN_H and CAN_L at the network ends; repair if open or out-of-spec.
- Use a scope or CAN monitor to confirm presence/absence of valid CAN frames and to spot noise, dominant state stuck, or irregular timing. Compare to a known-good waveform if available.
- If a specific module is absent from the bus, verify it has correct power and ground at its connector; check fuses and supply circuits.
- Isolate by disconnecting segments or removing aftermarket devices to determine if message returns.
- Inspect wiring for shorts to battery or ground by measuring resistance to ground/power with the module(s) powered off (follow manufacturer procedures).
- If wiring checks good and the module is powered but not communicating, consult OEM service info for software updates or reflash procedures before module replacement.
- Replace wiring/connector components only when proven faulty; replace modules only after verifying the fault follows the module (swap with known-good unit if applicable). Clear codes and retest on road or dynamic conditions to confirm repair.
Likely causes
- Faulty or unpowered module that transmits the expected message
- Open or high-resistance splice/connector in the specific bus branch
- Short to battery or ground on CAN_H or CAN_L causing invalid signals
- Missing or damaged network termination resistor(s)
- Software/calibration mismatch after module replacement or update
Fault status
Status
Inter-module communication — expected network message missing or invalid. A module detected absent or corrupted data on the vehicle communication network.
Repair difficulty: Medium
Diagnostic time: 1.5-3.5 hours
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