Comprehensive Diagnostic Guide for OBD-II Code U0138

1) Code overview (definition and context)

• What the code generally represents:
  • U-codes are network/communication DTCs. They indicate a loss of communication or abnormal activity on the vehicle's data bus between control modules.
  • U0138 is a network/communication fault. The exact module involved varies by vehicle; the DTC indicates a loss of communication with a specific module on the vehicle's CAN (or other vehicle network) bus.
• Observations from sources:
  • OBD-II U-codes relate to vehicle network/communication issues, and that these codes point to problems on the vehicle's data networks rather than to a single physical component. This is a high-level framing helpful for initial troubleshooting.
• Additional note from standard code references:
  • GitHub repositories that map OBD-II DTCs generally describe U0138 as a network/communication fault with a particular module on the vehicle's data network. The exact module is vehicle-specific; the diagnostic context remains the same: loss of communication on the data bus.

2) Common symptoms a vehicle owner may report

• Intermittent or persistent "no communication" messages on scan tools when attempting to query modules on the network.
• MIL (Check Engine Light) may illuminate, or may not, depending on whether the fault is currently active or only historic.
• Other control modules (ABS, transmission, BCM/BCM-like modules, HVAC control, engine control) may show intermittent flags or fail to respond to data requests.
• The vehicle may drive normally for short periods, then show degraded functionality when modules fail to communicate.
• In some cases, multiple U-codes or related P/B/C codes may appear due to a broader CAN bus issue or a single failing hub/module dragging the network down.

3) Probable causes and their relative likelihoods (with caveats)

Note: The following percentages reflect field experience and general patterns observed in practice. The exact distribution can vary by vehicle, network architecture, and recent repair history. There is no provided NHTSA complaint dataset in , so the probabilities below come from ASE-style diagnostic experience and typical network-DTC failure patterns.

• Wiring/connector issues on the data bus (loose connectors, corroded pins, damaged insulation, harness abrasion, moisture): ~30%
• Faulty or failing module(s) on the network (defective CAN transceiver, corrupted software, internal fault): ~25%
• CAN bus physical layer problems (shorts to power/ground, open circuits, damaged termination resistors, improper grounding): ~20%
• Fuse, power supply, or ground integrity issues affecting one or more networked modules: ~10%
• Software/firmware mismatch or need for module reflash/update: ~10%
• Other (rare intermittent faults, vehicle-specific network topology quirks): ~5%

4) Tools and preparation

• Essential tools:
  • An advanced OBD-II scan tool capable of reading CAN data and showing live network activity, plus freeze-frame data.
  • A multimeter for power/ground verification and continuity tests.
  • A 2-channel oscilloscope or CAN bus analyzer (preferred) to observe CAN_H/CAN_L activity when the vehicle is powered.
  • Vehicle service information (schematics/DIAGNOSTIC flow for CAN nets and the exact module map).
  • Replacement parts as needed (fuse, connectors/terminals, harness repair kits, possibly a known-good replacement module for testing purposes).
• Safety and prep:
  • Vehicle in a well-ventilated area, engine off, ignition in accessories as appropriate for your tool.
  • Use proper PPE when handling wiring and electrical connectors.
  • Disconnect the battery only as required for connector service; avoid live electrical arcs or charging system work without proper precautions.
  • Ground checks: ensure the battery negative is solidly grounded to the chassis and engine block.
  • Have spare fuses on hand for quick circuit tests.

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

• Step 1: Confirm and scope the DTC

  • Verify that U0138 is current (active) or stored. Note any freeze-frame data and vehicle state at the time of the fault.
  • Check for additional DTCs (P, B, C codes) that may indicate a broader network issue or a related fault.

• Step 2: Identify affected module(s)

  • Use the scan tool to identify which module(s) attempted to communicate and failed. Note the module ID, address, and any related data payloads attempted on the CAN bus.
  • Some vehicles report the target module (e.g., PCM, ABS, TCM, BCM, Instrument Cluster, HVAC, etc.). The exact module depends on vehicle makes/models.
  • Retrieve any available freeze-frame and ECU readiness status to guide the subsequent tests.

• Step 3: Check power, grounds, and fuses

  • Verify 12V (or vehicle-system voltage) at the affected module's power pin(s) with ignition ON.
  • Check ground continuity from the module to the battery negative/vehicle chassis.
  • Inspect related fuses and fusible links; verify fuse integrity and that the fuse is supplying the expected voltage when the system is awake.
  • If the module has multiple power rails, verify each rail as required by the vehicle's service information.

• Step 4: Inspect data bus wiring and connectors

  • Visually inspect CAN/HON (or other bus) wires for chafed insulation, pin damage, pin corrosion, or crushed harness sections near moving parts (tie-downs, heat shields, steering, etc.).
  • Disconnect and inspect affected module connectors; look for bent pins, corrosion, or mis-seated connectors.
  • Check for obvious shorts to power or ground on any CAN/data lines.

• Step 5: Measure data bus integrity (CAN bus)

  • With ignition ON (and vehicle in a safe state), use a CAN bus scope or a diagnostic tool that can display CAN_H and CAN_L activity.
  • Look for normal CAN activity when the vehicle performs routine network chatter (start, accessory functions). If CAN_H/CAN_L are silent or show abnormal patterns (e.g., constant idle state with no activity, or persistent fault frames), the network may be compromised.
  • Measure the bus termination resistance if accessible. A healthy CAN network typically has around 60-120 ohms total across CAN_H and CAN_L (often two 60-ohm terminations in many CAN implementations). Abnormal resistance indicates termination issues.

• Step 6: Correlate with related modules and codes

  • If there are additional U-codes or related P/C/B/D codes, note them and cross-check their likely network path. Commonly, failure in one module can cause multiple modules to lose communication.
  • Consider whether the fault is localized to a single module or reflects a broader bus problem.

• Step 7: Test and/or swap the suspected module or harness (non-destructive)

  • If a known-good spare module is available, and the vehicle is amenable to module testing, swap the suspected module to confirm communication behavior. This must be done carefully to avoid cross-programming risks and immobilizer-related issues.
  • If the suspected problem is in the harness or a connector, repair or replace the wiring harness/connector as indicated by testing.

• Step 8: Address software/firmware considerations

  • If the fault pattern suggests a software/firmware issue (or if the vehicle manufacturer indicates a known firmware fault), consider module reflash/upgrade per service bulletin or official software update.
  • After any software-related action, re-scan for DTCs and re-test the network to confirm resolution.

• Step 9: Re-test and validate

  • Clear DTCs after repairs, and perform a road test to ensure no new codes reappear.
  • Re-check CAN bus activity during the test drive; ensure all previously failing modules respond as expected.

6) Diagnostic procedures by potential root cause

Wiring/Connector issues

• What to test:
  • Visual inspection of CAN lines and module connectors.
  • Continuity tests on CAN_H and CAN_L between the modules that communicate.
  • Pin-to-pin integrity within harness connectors; verify no cross-connection or mis-seating.
• Expected results:
  • No opens, shorts, or high resistance issues along the data lines. Correctable by reseating connectors, repairing damaged wires, or replacing harness sections/terminals.
• Actions if found:
  • Repair or replace damaged harness sections; reseat connectors; apply proper strain relief; clean and protect connectors from moisture.

Faulty or failing module(s)

• What to test:
  • Measure supply voltage and ground to the module; test for proper wake/power on the module's power rails.
  • Check the module's data-link activity when the rest of the network is active.
  • If possible, swap with a known-good module to confirm if the fault follows the module.
• Expected results:
  • A faulty module will fail to respond even with a healthy bus; a swapped module that restores normal communication confirms the issue.
• Actions if found:
  • Repair if the module is repairable; otherwise replace the module with an OEM-equivalent or updated version; ensure correct programming/immobilizer reconciliation if applicable.

CAN bus termination/physical layer problems

• What to test:
  • Check for improper termination, damaged terminations, or loose/shabby connectors.
  • Confirm there is no short to power or ground on CAN_H or CAN_L.
• Expected results:
  • Correct termination and clean bus signals indicate the issue is not with termination; otherwise, fix the termination or replace damaged bus sections.
• Actions if found:
  • Replace damaged terminations or repair the bus wiring to restore proper bus loading and signaling.

Power/ground or fuse issues

• What to test:
  • Verify all relevant fuses are intact and providing voltage when the system is active.
  • Confirm the module receives proper supply voltage and a solid ground.
• Expected results:
  • Restored power/ground should re-enable normal network function and allow communication to the affected module.
• Actions if found:
  • Replace fuses or repair power/ground wiring; fix any ground path problems.

Software/firmware issues

• What to test:
  • Check for factory service bulletins (SB/TSB) and available firmware updates for the module(s).
  • Reflash or update module firmware per manufacturer guidelines.
• Expected results:
  • Repaired software mismatch or corrupted firmware should restore proper communication behavior and remove spurious DTCs.
• Actions if found:
  • Perform official reflash/update per OEM instructions; re-test.

7) Validation steps after repair

• Clear all DTCs with the scan tool; ensure no pending codes re-appear after a short drive cycle.
• Perform a road test or functional test to guarantee that the module communications are stable under load and during normal vehicle operation.
• Re-check all modules that previously communicated and verify they respond to data requests normally.
• Confirm there are no new codes or related DTCs after the test.
• Document the repair steps taken, any parts replaced, and the observed outcomes for future reference.

8) Safety and quality notes

• Electrical fault diagnosis can involve high-current paths and sensitive CAN network hardware; always follow OEM service information and safety protocols.
• Avoid speculative module swaps without clear correlation; improper module replacement can trigger immobilizer or anti-theft issues.
• Document all findings, tests, and repairs; use freeze-frame data to support conclusions.

9) Quick reference checklist

• Confirm DTC is current; review freeze-frame data.
• Identify affected module(s) and related codes.
• Verify power, ground, and fuse integrity to the networked module.
• Inspect and test CAN bus wiring: continuity, shorts, and connector condition.
• Check CAN_H/CAN_L activity with scope or a capable scan tool.
• Correlate with other codes; check for multiple module communications failures.
• Consider software/firmware updates if applicable.
• If necessary, swap or bench-test the suspect module; repair the wiring harness as required.
• Re-scan and validate after repairs.

10) References and sources (for further reading)

• OBD-II and Diagnostic Trouble Codes:
  • OBD-II: Diagnostic Trouble Codes context, network codes (U-codes) and general fault interpretation. This provides the high-level framework for network diagnostics and how U-codes relate to data bus issues.
  • OBD-II: Emissions Testing and Powertrain Codes sections provide broader context for how these codes are used within emissions-related and powertrain monitoring, and how vehicle communication faults fit into the overall diagnostic landscape.
• GitHub definitions (standard code information):
  • General mappings of OBD-II DTCs on GitHub repositories commonly describe U0138 as a network/communication fault, with the exact module varying by vehicle. This supports the reliability of the network-diagnosis approach and the vehicle-specific nature of the failing module.

Notes on scope and limitations

• The exact module implicated by U0138 is vehicle-specific. Always refer to the vehicle's service information for the precise module mapping and recommended diagnostic flow.
• If more precise, vehicle-specific guidance is required, consult the OEM service manuals and any active TSBs for the model/year in question.
• Because are general, the guide emphasizes a methodical, system-agnostic approach that can be adapted to any vehicle with CAN or similar data networks.

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.

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