Comprehensive diagnostic guide for OBD-II code U0387

• Code class and general meaning: U codes are network/communication-related trouble codes in OBD-II. They indicate a fault on the vehicle data bus or between control modules. This is consistent with the OBD-II framework described in Wikipedia's Diagnostic Trouble Codes and Powertrain Codes sections (e.g., U-codes are in the network category) and the broader discussion of how OBD-II monitors and reports faults. See: Wikipedia - OBD-II sections on Diagnostic Trouble Codes and Powertrain Codes.
• OEM-specificity: The exact description and the module involved for U0387 are OEM-specific. The generic guidance below focuses on network-bus fault symptoms and typical diagnostic steps that apply across vehicles.
• Standard code information: GitHub definitions and community references generally categorize U-codes as vehicle network communication faults (CAN, LIN, etc.) and indicate that U0387 is a network-related issue described by OEMs. The guide below uses that framework.
• Symptom/causality probabilities: No explicit NHTSA complaint data for U0387 is provided . When numbers are given, they are based on typical field experience (ASE-level diagnostic practices) and general patterns discussed in the network-code context.

1) What U0387 means (code concept and scope)

• U0387 is a network/communication DTC. It signals a fault on the vehicle's data bus or between one or more control modules that rely on bus communication.
• The exact module involved is OEM-specific; in many cases U0387 describes a lost or failed communication with a specific module or with the vehicle network in general.
• Because U-codes are network-related, a single U0387 often appears alongside other U-codes (or repeated communications faults) rather than as a lone, fully diagnostic P-code issue.

2) Common user-reported symptoms (informing symptom description)

• Check engine/maintenance lights may be on or flashing intermittently due to multiple modules reporting faults.
• Scan tool shows U0387 and may display other U-codes or U0100/U0101-type messages (lost communication with a module or network bus errors).
• Intermittent or scattered module behavior: gauges, BCM/TCM transmission status, ABS/traction control, body control functions, or instrument cluster showing "no communication" or erratic operation.
• In some cases, systems appear to operate intermittently, then revert to normal until the next network fault occurs.

Note: The above symptom patterns are consistent with general OBD-II network fault behavior and the role of U-codes in signaling bus problems. The sources describe U-codes as network issues and emphasize that OEM-specific details matter for exact diagnosis.

3) Probable causes (with practical likelihood, field experience)

• Loose, damaged, corroded, or poorly seated CAN/LIN/L-bus connectors and wiring harnesses (including grounds near modules): ~40%
• Faulty module(s) on the network, such as the gateway/module that manages inter-module communication, or a failed/locked ECU: ~25%
• Missing or failed termination or abnormal bus resistance (especially on older networks or in vehicles with aftermarket wiring): ~15%
• Software/firmware mismatch or corruption in one or more ECUs that prevents proper bus communication (requiring reflash or updates): ~10%
• Electrical issues affecting power to the network (insufficient battery/alternator voltage, grounding problems) causing intermittent comms: ~10%

These percentages reflect a general trend for network faults where wiring/connectors and a faulty bus controller or gateway are the most common culprits, followed by module/software issues. They are not OEM-specific and should be treated as guidance rather than exact probabilities.

4) Essential tools and preparation

• Defender tools:
  • OBD-II scan tool capable of reading CAN bus data and showing multiple U-codes (a generic scan tool may be enough for initial diagnosis; OEM- or dealer-grade tools are preferred for module-level communication tests).
  • A digital multimeter (DMM) to test continuity, resistance, and supply voltages on ignition sense, 5V references, and power/ground rails.
  • An oscilloscope or CAN bus analyzer (preferred) to observe CAN_H and CAN_L signals, bit timing, and bus activity.
• Vehicle prep:
  • Ensure battery voltage is stable (ideally 12.6V+ with engine off; during tests, 13.5-14.8V under run).
  • Check for corrosion on battery terminals, grounds, and known chassis/engine ground points.
  • Note any recent work that could have disturbed wiring, connectors, or module mounting.
• Documentation:
  • Vehicle-specific service information (OEM diagnostics) to identify which modules participate on the CAN/LIN bus and which module is the gateway/diagnostic interface.
  • Freeze-frame data and any data stream snapshots from the scan tool.

5) Diagnostic flow (step-by-step approach)

Follow a structured flow to identify the source of the network fault:

Step 1 - Confirm and scope

• Confirm U0387 with a reputable scan tool and note any accompanying DTCs (especially U0100/U0101, U1121, or other U-codes).
• Gather freeze-frame data to see the vehicle conditions when the fault occurred (voltage, engine status, etc.).
• Note OEM-specific fault description if provided by the OEM's codes.

Step 2 - Check mechanical and power conditions

• Inspect battery condition and charging system. Low voltage or unstable voltage can cause network faults.
• Inspect all relevant grounds (engine block ground, chassis grounds, EMI grounds near ECUs).
• Inspect fuses related to the vehicle's data bus and each module.

Step 3 - Visual network inspection

• Inspect CAN/LIN harnesses and connectors for damage, loose pins, corrosion, or pin-twist symptoms.
• Look for aftermarket wiring or recent wiring repairs that could have introduced interference or poor terminations.

Step 4 - Identify the affected modules by correlation

• Use the scan tool to see which modules are reporting issues or not responding. Note if multiple modules are unavailable.
• If possible, identify the OEM-described module(s) that are not communicating. This helps determine whether the fault is widespread (bus, gateway) or localized (a single module).

Step 5 - Bus integrity checks

• Measure bus resistances: typical CAN bus termination is around 60 Ω total (two 60 Ω resistors in parallel is ~60 Ω). Verify there is proper termination and no short to ground or power.
• Look for improper terminators or parallel terminations that can upset bus performance.
• If possible, check CAN_H and CAN_L differential signals with an oscilloscope:
  • You should observe a differential waveform with toggling frames. No waveforms or indistinct signals suggest a bus problem or multiple modules fighting the bus.

Step 6 - Module-level testing and isolation

• If you can isolate a suspect module, try performing a controlled test (e.g., disconnect a suspected module with the ignition OFF, then re-check bus activity and other modules). Do not disconnect modules in a way that creates a loss of essential system function without OEM guidance.
• In vehicles with gateway modules, assess the gateway's status and ability to route messages between networks. A faulty gateway commonly causes widespread U-codes.

Step 7 - Firmware/software considerations

• Check for any available OEM software/firmware updates for the ECUs involved, especially gateway, BCM, or other central controllers.
• If the vehicle has aftermarket or counterfeit modules, consider module compatibility and reflash needs.

Step 8 - Reproduce and confirm

• Clear DTCs and monitor the vehicle under safe, controlled conditions.
• Run functional tests (e.g., powertrain operation, instrument cluster operation, ABS/traction control if applicable) and observe whether the U0387 recurs or if other network-related codes appear.

6) Repair actions by scenario

• Wiring/connectors issue:
  • Repair damaged harness sections, clean and reseal connectors, re-pin as necessary, and apply proper dielectric grease where appropriate.
  • Re-seat all module connectors with the ignition OFF; ensure pins are straight and locked.
• Ground and power issues:
  • Repair or replace corroded grounds; ensure solid engine/chassis grounds; repair battery connections; ensure stable 12V supply during operation.
• Faulty module or gateway:
  • Replace or reflash the affected module per OEM service information.
  • If the gateway or central router/module is at fault, replace with OEM-approved unit and perform any required reprogramming.
• Software/firmware updates:
  • Apply OEM-released ECU firmware updates or calibrations to correct communication logic or protocol handling.
• Bus termination or layer issues:
  • Correct termination resistors if missing or incorrect. Ensure proper bus topology and absence of shorts to power/ground.

7) Verification and validation

• After repairs, clear DTCs and re-run the vehicle through normal operating conditions; verify that U0387 does not reappear.
• Confirm that all modules that previously reported "no communications" are now communicating on the network.
• Use the scan tool to observe live data from multiple ECUs; ensure network frames are present and error-free without recurring bus faults.
• If OEM tool testing is available, run OEM network diagnostics to verify bus health and module responsiveness.

8) Safety considerations

• Work with the ignition off and the battery disconnected when performing physical inspections on connectors and harnesses to avoid short circuits.
• When working with the vehicle's data bus and electrical system, avoid exposing components to moisture or conductive contaminants.
• If you are not sure about the OEM-specific diagnostic steps, consult the official service information for your vehicle.

9) Quick-reference summary

• What it is: U0387 = network/communication fault on the vehicle's data bus (OEM-specific description).
• Primary suspects: loose/damaged wiring, bad connectors, failed gateway or ECU, bus termination issues, or firmware problems.
• Most productive first steps: verify power/ground stability, inspect harnesses/connectors, identify which modules are failing to communicate, and inspect bus topology/termination.
• Verification: after repair, clear codes, cycle the ignition, and re-test with a live data scan to confirm the network is healthy and modules are communicating.

10) References and corroboration

• OBD-II Diagnostics and Code classes (including U-codes as network fault codes and the general structure of diagnostic trouble codes; Powertrain Codes section helps explain code families and where U-codes fit in the overall scheme). These provide the foundational understanding of network DTCs and their placement within OBD-II code taxonomy.
• General understanding of U-codes and network communication in OBD-II.
• GitHub definitions (community-based) corroborate that U-codes are vehicle network communication faults and that U0387 is within that family, described OEM-specifically. Use OEM service information for exact factory descriptions.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II

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

---