Comprehensive diagnostic guide for OBD-II code U0104

Definitions and context

• What U0104 generally means: In standard OBD-II practice, U-codes are network/communications codes. U0104 indicates a loss of communications with a particular control module on the vehicle's data bus. The exact module involved depends on the vehicle and its network topology (CAN or other data buses). The concept is "lost or no communications with a specified module." For general understanding, see the OBD-II Diagnostic Trouble Codes overview.

  • Wikipedia (OBD-II): DTCs are generated by on-board monitors; U-codes are associated with vehicle network/communication issues as part of the broader DTC landscape (Diagnostic Trouble Codes; Powertrain Codes sections). This provides the framework that U0104 is a network/communication fault rather than a direct sensor fault.
  • The "Powertrain Codes" section reinforces that OBD-II uses a common code system to report issues across powertrain and connected control modules.
  • NHTSA complaint data can illustrate real-world symptoms that accompany drivability concerns, though the specific complaint shown does not necessarily map to U0104.
  • GitHub definitions (in practice): Standard OBD-II DTC naming uses UXXXX for network issues; U0104 is interpreted as a lost communication with a control module on the vehicle network. The exact module depends on the vehicle and its wiring diagram.

• Practical implication for technicians: A U0104 usually points to a data-bus/communication fault rather than a single sensor or actuator being defective. Isolating the exact module affected requires vehicle-specific wiring diagrams and live data from the vehicle's CAN (or other) network.

Symptoms and symptom patterns (what you might observe)

• Common symptom themes for U0104 (non-vehicle-specific, network-centric):
  • Intermittent or persistent loss of communication with one or more ECUs (e.g., ECM/PCM, TCM, ABS, BCM, Instrument Cluster).
  • Multiple modules reporting U-coded or no-communication states, sometimes accompanied by warning lights or degraded drivability.
  • Scan tool shows "no communication" or "not available" for certain modules; freeze-frame data may show prior faults or normal values before the loss occurred.
  • In some cases, intermittent abnormal vehicle behavior may occur if critical modules lose data or fail to coordinate (e.g., transmission, stability, or engine control responses).
• Real-world symptom example: A complaint described drift into another lane and abrupt vehicle stoppage in a 2017 Honda Pilot. This illustrates how real drivability/safety symptoms can accompany vehicle electronics issues, but it is not a confirmation that U0104 was present. It demonstrates the kind of broad safety-critical behavior that can accompany electronic faults. Use this to inform your suspicion that network faults can contribute to unpredictable vehicle behavior.

What can cause U0104 (probable causes, with bias toward field experience)
Note: There is limited public NHTSA complaint data specifically for U0104. Use these probabilities as informed guidelines rather than definitive statistics.

• CAN/vehicle-network faults (40-60%)
  • Damaged, pinched, corroded, or unplugged data-bus wiring between modules.
  • Loose or contaminated connectors, bent pins, or water intrusion at ECU plugs.
  • Faulty termination or improper network topology (rare but possible in vehicles with improper aftermarket wiring).
• Faulty or failing control modules (20-30%)
  • ECM/PCM, TCM, ABS/DSC controller, BCM, or other ECUs intermittently failing to respond on the bus.
  • Aftermarket modules or re-flashing/tuning that altered network behavior.
• Power supply and grounding issues (10-20%)
  • Insufficient battery voltage or poor ground integrity that disrupts module wake-up and bus activity.
  • Fuse issues or intermittent power to one or more modules on the data bus.
• Software/Calibration anomalies (5-15%)
  • Outdated firmware or software incompatibilities between modules causing misalignment on the bus.
  • Recent software updates or reprogramming that didn't complete cleanly.
• External electrical noise or interference (low probability, variable by vehicle)
  • Aftermarket accessories or faulty alternators can introduce bus noise, especially on older networks.

Diagnostic Approach

Goal: Confirm a network communication fault and identify the faulty module or wiring causing U0104, then repair and verify.

Phase 1 - Initial verification

1) Confirm the DTC and capture data

• Use a compatible scan tool to confirm U0104 is present and note any related U-badges (e.g., U0100, U0101, U0102, U0103, etc.) and any accompanying P-codes.
• Record freeze-frame data and any present vehicle conditions (engine on/off, gear, speed, battery voltage, and ambient temperature).
• Note whether the vehicle has multiple ECUs reporting no communication.

2) Check basic vehicle health and data integrity

• Verify battery voltage is healthy (typically 12.5-12.9 V at rest; under cranking, voltage should not collapse significantly).
• Check for obvious power/ground issues: loose grounds, corroded battery terminals, poor chassis grounds.

Cited context: DTCs originate from on-board monitors; U-codes signal network/communication issues.

Phase 2 - Correlation with other codes and symptoms

3) Look for related codes and symptoms

• If there are multiple U-codes or any PERR (pending) codes, note their module associations.
• Check for any live data that indicates which modules are actively communicating and which are not (scan tool CAN monitor, DM1/DM20 messages if supported by tool).

4) Visual/physical inspection of the network

• Inspect CAN/data-bus harnesses, high-contrast connectors, and module connectors for missing seals, moisture intrusion, or cosmetic damage.
• Inspect for aftermarket wiring, alarm/remote start wiring, or added devices that could interfere with CAN wiring.

Phase 3 - Electrical system checks (power, grounds, and network)

5) Power and grounds

• Confirm stable 12V supply to modules (check fused power circuits and battery feed to the ECU(s)).
• Check for good grounds at the ECU mounting points and at the vehicle chassis.
• Inspect for known vehicle-specific grounding issues that could affect multiple ECUs.

6) CAN bus physical layer checks

• If you have access to a CAN bus scope or logic analyzer, check CAN High (CANH) and CAN Low (CANL) signals for proper differential voltage and stable signal levels during engine-start and normal operation.
• Check for proper termination: typical CAN networks have 120-ohm terminators at each end; ensure there are no missing or duplicate terminators.
• Look for shorts to voltage or ground on CAN lines, and inspect for consistency of wire routing (avoid sharp bends or heat sources).

7) Module-by-module communication assessment

• Identify the modules that should be on the network in your vehicle (ECM/PCM, TCM, ABS, BCM, Instrument Cluster, etc.).
• With the scan tool, attempt to access each module's diagnostics or read data. Note which modules fail to respond and whether any modules respond intermittently.
• If a single module consistently does not communicate, suspect that module or its power/ground feed. If multiple modules fail, suspect CAN bus or a common feed/ground or a critical gateway module.

Phase 4 - isolation strategy

8) Isolate the network to identify the faulty area

• If possible, perform a controlled disconnection test:
  • Disconnect suspected module harnesses one at a time and re-scan for U0104. If the code clears with one module disconnected, that module or its harness is a likely source.
  • If the code reappears after reconnecting, re-check the connections and harness integrity for that module.
• If disconnecting modules does not clear U0104, inspect the primary CAN backbone wires for damage or short circuits, and check the vehicle's network gateway (module that bridges networks if present).

9) Consider software and vehicle-specific factors

• If modules have available firmware updates, consider reprogramming or reflash as needed, after ensuring power stability and proper procedure.
• Ensure aftermarket electronics are not interfering with the CAN network; temporarily remove non-factory equipment if feasible to test.

Phase 5 - Repair options and preventive steps

10) Typical repair pathways

• Wiring/harness repair: fix damaged, abraded, or corroded CAN wires; reseat and reseal connectors; replace damaged terminals.
• Module repair or replacement: replace or reprogram a faulty module that fails to communicate reliably; ensure proper reflash/compatibility with other modules.
• Power/ground remediation: fix ground straps, clean grounds, repair power feeds to affected modules; replace fuses if necessary.
• Aftermarket removal: remove interfering devices or update wiring to restore proper bus integrity.
• Software updates: apply OEM-released firmware/software updates and recheck CAN communications after completion.

11) Verification after repair

• Clear DTCs and perform a road test, ideally under a controlled environment and with the scanner observing live data.
• Confirm that U0104 does not reappear and that all modules once affected are communicating normally.
• Verify there are no new DTCs and that related subsystems (e.g., engine, transmission, ABS, instrument cluster) show normal data flow.
• Confirm battery voltage remains steady during the test drive; monitor for any intermittent faults during varying load.

Safety and risk considerations

• A network fault can affect essential safety systems. Treat a U0104 seriously; unexpected module behavior or power loss can impact drivability and safety.
• When working with live vehicle networks, avoid creating new shorts; disconnect battery only as required and follow proper procedure to avoid accidental fault propagation.
• If unsure about module identity or wiring diagrams for the specific vehicle, obtain OEM wiring schematics and vehicle service information before performing invasive diagnostics or component removal.

How to document and communicate results

• Record all observed codes (U0104 and any related codes), live data snapshots, and module communication statuses.
• Document the exact module(s) tested and the outcomes of any isolation tests.
• Provide a clear repair narrative: the fault suspected, the repair performed, and the verification results.
• Note if software updates or recalibration were performed and the outcomes.

Summary

• U0104 strongly suggests a data-bus/communication fault rather than a single-sensor failure. Vehicle-specific wiring diagrams and a methodical approach to the CAN network are essential to locate the fault.

• Expect multiple possibilities: wiring/connectors, a faulty module, power/ground integrity, or software issues. A disciplined diagnostic flow helps isolate the cause.

• Use the vehicle's data stream and module response behavior to guide your isolation strategy; do not rely solely on one symptom or a single DTC.

• Wikipedia - OBD-II: Diagnostic Trouble Codes and Powertrain Codes

  • Provides the framework that U-codes are network/communication-related and that DTCs are generated by on-board monitors.

• Real-world complaint context

  • Complaint text illustrating how real-world symptoms can present as drivability/safety concerns (e.g., an unintended lane drift and sudden stoppage). Use this to understand symptom breadth; it is not a direct mapping to U0104.

• General code information

  • Widely used online definitions and references for DTC naming conventions, including UXXXX codes for network communications and U0104's role as a lost-communication code on the vehicle network.

Note on data-driven probabilities

• Specific NHTSA data for U0104 is not provided in the available sources. The probability estimates for causes above are based on typical field experience with U-codes and common network fault patterns, as described in the diagnostic approach and general DTC discussions. If additional NHTSA data for U0104 becomes available, .

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.

---