Comprehensive diagnostic guide for OBD-II code U0143

Important Notes

• U codes are network/communications fault codes (OBD-II). They indicate lost or impaired communication on the vehicle's data networks (CAN, LIN, etc.). Exact module names and network topology vary by manufacturer, so interpretation of which module is "lost" can change between vehicles. Source notes: OBD-II diagnostics basics and the network/code concept are described in Wikipedia's OBD-II sections; standard code definitions are often cataloged in GitHub repositories that define U0143 as a network communication loss. The exact module affected is typically manufacturer-specific.
• Real-world signal: A documented NHTSA user complaint describes a U0143 network code presenting as a "Trailer disconnected" warning/chime, sometimes when no trailer is connected. The dealer reportedly cleared the code and test-driven without a repeat failure. This illustrates how network codes can trigger seemingly unrelated warnings and why a systematic network diagnostic is needed.
• Summary definition (general): U0143 indicates a lost or disrupted communication on the vehicle's data network with a control/module, i.e., a network-level fault on the CAN (or other) network. Manufacturer-specific interpretation may point to BCM, gateway/module, IPC, or other networked controllers.

1) Symptom profile and patient history

• Primary symptom described in verified NHTSA data: intermittent warning message such as "Trailer disconnected," with a chime, appearing even when no trailer is attached.
• Accompanying code: U0143 (network/code indicating lost/erratic network communication).
• Observations to note during diagnosis:
  • Warning may appear sporadically and across multiple driving sessions.
  • No persistent physical trailer connection required for reproduction.
  • Other U-codes or vehicle warning lights may appear concurrently if the network is unstable.
• Practical takeaway: Because U0143 relates to network communications rather than a single component, symptom clusters can be broad (instrument cluster, telematics, trailer harness, body modules, gateway, etc.). This requires systematic network testing rather than component replacement alone.

2) Likely causes and their estimated likelihood

Note: Based on the single NHTSA complaint we have, precise, statistically robust probabilities cannot be established. The following probabilities are provided as educated estimates with caution and should be confirmed by data from additional incidents if available.

• Intermittent CAN bus wiring/grounding issue (most likely in a symtomatically network-driven code)

  • Estimated likelihood: 40-60%

• Faulty/aging body control or gateway module (BCM, gateway, IPC, or other networked controllers)

  • Estimated likelihood: 20-30%

• Software/firmware issue or stale calibration in networked controllers

  • Estimated likelihood: 10-20%

• Aftermarket devices or improper wiring converging on the CAN bus

  • Estimated likelihood: 5-10%

• Other causes (short to power/ground on CAN lines, power supply instability, environmental factors)

  • Estimated likelihood: 5-15%

This diagnostic guide was generated using verified reference data:

• NHTSA Consumer Complaints: 1 real-world reports analyzed
• Wikipedia Technical Articles: OBD-II

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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