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Home / DTC / U2004 — Package of auxiliary switches

U2004 — Package of auxiliary switches

Detailed page for trouble code U2004.

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U2004

LAND ROVER U — Network/User

Package of auxiliary switches

Brand: LAND ROVER
Type: U — Network/User
Views: UK: 4 EN: 5 RU: 6
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Page language: EN

Causes

  • Open/short in wiring to the auxiliary switch pack (power, ground or CAN lines)
  • Poor connector contact (corrosion, bent pins, water intrusion)
  • Blown fuse or intermittent power supply to the module
  • CAN bus fault (short to battery/ground, missing termination, high bus error rate)
  • Faulty auxiliary switch pack/module electronics
  • Incorrect or corrupt module software/calibration

Symptoms

  • One or more auxiliary switches do not operate or are intermittent
  • Warning or message in instrument cluster indicating auxiliary switch fault
  • Related accessories (lights, pumps, relays) not responding to switch commands
  • DTC U2004 stored in vehicle networked modules
  • Erratic behavior of vehicle electrical accessories fed via the auxiliary pack

What to check

  • Read all network DTCs and freeze frame data with a Land Rover–compatible scan tool
  • Check that the auxiliary switch pack is present and reported on the CAN network (module ID)
  • Visually inspect connectors and harness for damage, corrosion, moisture or pin push-out
  • Confirm fuses/power supplies to the auxiliary pack are good and supply correct voltage
  • Measure and compare CAN HIGH / CAN LOW voltages at the module with ignition on
  • Check for CAN bus error frames and message frequency with a scope or diagnostic tool

Signal parameters

  • CAN bus idle voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (recessive)
  • CAN dominant state: CAN_H rises to ≈ 3.5 V and CAN_L drops to ≈ 1.5 V
  • Bus termination: combined 60 Ω equivalent across CAN_H and CAN_L at rest
  • Module should respond to its CAN ID and transmit expected status frames when commands sent
  • Message frequency: auxiliary switch status messages should appear at defined intervals (check manufacturer data)
  • Battery supply: expected 12 V (nominal) to module with ignition states as specified by wiring diagram

Diagnostic algorithm

  1. Use a Land Rover–capable scan tool to record U2004, related network DTCs and freeze frame. Note ignition state and time.
  2. Clear DTCs and attempt to reproduce fault by operating auxiliary switches while monitoring live data and network messages.
  3. Visually inspect connector at auxiliary switch pack and associated harness for damage, corrosion or water ingress. Repair as required.
  4. With ignition on, check supply fuse and measure battery voltage at the module power pin and ground continuity to chassis.
  5. Measure CAN_H and CAN_L at the module connector: confirm idle voltages, check for missing/shorted line or abnormal differential.
  6. Use oscilloscope or CAN bus analyzer to observe message frames and error counters; look for bus errors, missing transmissions or a noisy node.
  7. Isolate the module: disconnect the auxiliary switch pack and verify whether the CAN bus returns to normal (watch for reduced error rate).
  8. If wiring and bus are good, check module coding and software level; reflash or reconfigure per factory procedure if update available.
  9. If module fails bench or scope tests or remains unresponsive after correct coding/flash, replace the auxiliary switch pack and verify repair.
  10. After repairs, clear DTCs and perform a full function test of all auxiliary switches and monitor the network for recurrence.

Likely causes

  • Intermittent or missing power/ground to the auxiliary switch pack
  • Damaged or loose connector/CAN wiring at the module
  • High error rate on CAN bus caused by shorted device or poor termination
  • Failed auxiliary switch pack module requiring replacement or reflash

Fault status

⚠️ Status
U2004 — Communication/functional fault reported for auxiliary switch pack. Check power/ground, connectors and CAN network; module may be non-responsive or sending invalid messages.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-3 hours

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