Comprehensive Diagnostic Guide for OBD-II Code U0391

Disclaimer on meaning

• U codes are vehicle network/communication diagnostics. They are often manufacturer-specific and can point to loss of communication with a particular control module on the vehicle's data bus. The exact module involved and the documented definition for U0391 can vary by manufacturer and model.
• What this means for diagnostics: treat U0391 as a network/communication fault that likely involves the CAN (or other) data bus backbone, a misbehaving module, or a power/ground issue affecting multiple controllers. OEM service information or GitHub-style code references (manufacturer-specific definitions) should be consulted to identify the exact module involved for a given vehicle.
• Context: Wikipedia's OBD-II overview notes that DTCs are generated by on-board diagnostics and that U-codes denote network/communications problems. The Powertrain Codes section clarifies that U-codes fall under the broader DTC framework used in OBD-II.

What you may notice (symptoms informed by typical network/communication faults)

• Check Engine Light/MIL on with U0391 stored.
• Intermittent or persistent "no communication" between the vehicle's ECUs; multiple modules may report not communicating.
• Dash/instrument cluster data may be inconsistent or freeze, or gauges may behave unexpectedly.
• Other DTCs (P, B, C) may appear, often related to modules that rely on bus communication.
• Various drivability concerns may accompany the problem, such as intermittent starting, warning lights, or messages on a scan tool about lost/timeout communications.

Probable Causes

• Loss/interruptions on the data bus or wiring faults (40%)
• A module that is unpowered, miswired, damaged, or faulty (25%)
• Power/ground issues, fuses, battery/alternator supply problems affecting one or more ECUs (10%)
• Software/firmware or ECU re-programming issues, or incompatible/incorrect software versions (10%)
• Electrical noise, aftermarket devices, or connector/ground corrosion causing intermittent bus contention (5%)
• OEM-specific interpretation can shift these weights; always confirm with vehicle-specific service information.

Diagnostic Approach

1) Safety and initial checks

• Ensure the vehicle is in a safe state for diagnostic work.
• Verify battery condition and charging: check system voltage (12.6-14.4 V when running), test for voltage drop during load, and inspect battery terminals for corrosion.
• Inspect fuses and relays related to data communication and power to ECUs (look for fuses labeled CAN, data link, BCM, BCM/TCM/ECM power, etc.). Some vehicles use dedicated CAN or data-link fuses.

2) Confirm code behavior and data

• Use a capable scan tool to verify U0391 is current (not historical) and to view freeze-frame data. Note any accompanying codes (P/B/C) and any "module not responding" messages.
• Clear the DTCs if appropriate and re-scan to see if U0391 returns, and whether other codes appear.
• Check the vehicle's network map (if your tool provides it) to see which modules are reporting communications errors or timeouts.

3) Map the network and identify the suspect area

• Look for which controllers show as not communicating (ECM/PCM, TCM, ABS, instrument cluster, BCM, RAD/infotainment, body controllers, etc.).
• If multiple modules fail to respond, suspect a backbone/physical layer issue (CAN High/Low wiring, a damaged splice, or a major grounding issue).
• If only one module is "not communicating" while others are OK, suspect that module's power/ground or its interface hardware, or a direct wiring issue to that module.

4) Inspect the physical data bus and related wiring

• Visually inspect CAN/CAN-like harnesses for damage, pin deformation, corrosion, or pin fallout at connectors.
• Check for aftermarket devices (alarm systems, infotainment upgrades, race telemetry boxes, amplifiers, or radio amplifiers) that could inject noise or opportunistically load the bus.
• Inspect grounds and mounting points for the modules involved; verify the main CAN shield/ground connections, and ensure proper chassis grounding.

5) Verify power and grounds to suspect modules

• Use a wiring diagram (OEM or service manual) to identify power and ground pins for modules on the network.
• Measure supply to suspect modules with the ignition ON and engine running (where applicable). Look for voltage drops on power pins and verify ground continuity to chassis and to the battery negative.
• Pay particular attention to shared power rails or fused feeds that can cause one or more ECUs to reset or fail to initialize.

6) Test the data bus integrity

• If you have the equipment, check CAN High and CAN Low with a multimeter or oscilloscope to confirm normal differential signaling, proper termination, and absence of short to power/ground.
• Look for bus contention (two devices driving the bus simultaneously) or excessive noise.

7) Isolate by module

• If practical, selectively disable or disconnect suspected modules (one at a time) and re-scan to see if the network fault resolves or a different module loses communication.
• If disconnecting a module restores the network, focus further on that module's interface, power/ground to that module, or its internal fault.

8) Check for software/firmware issues

• Check for OEM service bulletins (TSBs) related to CAN bus or module communication for your vehicle.
• If available, update or re-flash the firmware of suspect modules using OEM tooling. Ensure the vehicle is in the correct programming compatibility state before programming.

9) Validate and road-test

• After repairs, re-scan to confirm U0391 does not reoccur.
• Perform a road test to ensure that communication remains stable across modules and under driving conditions (RPM, load, temperature, etc.).
• Confirm that any related DTCs clear and do not re-appear under normal operation.

Module-specific considerations (guidance, not vehicle-by-vehicle)

• Controller that commonly triggers network faults: ECM/PCM, TCM, ABS/DSC/ESC, Instrument cluster, BCM, gateway/module controllers. The exact modules involved depend on vehicle architecture and which modules rely on the bus.
• Some vehicles have a gateway module that partitions CAN segments; a fault in the gateway can produce broad "not communicating" symptoms across several modules.

Tests and verification plan (practical checklists)

• Electrical tests:
  • Battery/alternator check; voltage stable during load.
  • Fender-wender test to ensure no ground loops or swapped ground paths exist.
  • Visual inspection of all CAN and data-link connectors; reseat connectors if needed.
• Network tests:
  • CAN High/Low voltage checks and differential signal verification.
  • Bus scanning: confirm which modules are online vs. offline.
  • Isolation tests: disconnect suspect module(s) and observe changes in network communication.
• Functional tests:
  • Confirm that the engine starts and runs under normal operation; confirm transmission functions if TCM is on the bus.
  • Verify that instrument cluster and other dashboards display correctly once communication is restored.

Repair paths (typical)

• Wiring/connector fixes: repair damaged wires, replace damaged connectors, fix ground straps, re-seat or replace gateway/bus connectors as indicated.
• Module-level fixes: replace or re-seat faulty module(s); verify proper power/ground and firmware version; re-flash with OEM software when required.
• Power/ground system fixes: repair or replace faulty battery, wiring harnesses, fuses, or grounding points affecting the data bus.
• Aftermarket interference: remove or relocate any non-OEM devices found on the data bus path to eliminate noise and bus contention.
• OEM service and programming: apply recommended TSBs; perform module reprogramming if indicated by OEM.

Post-Repair Validation

• Re-scan and confirm U0391 does not return; check for any new codes.
• Validate network integrity across modules; ensure all modules show as online.
• Conduct functional tests for affected systems (e.g., engine start, transmission shift, instrument cluster operation).
• Road-test to verify stability of CAN communication under real-world conditions.

Safety Considerations

• Do not arbitrarily disconnect airbag-related modules or any controller near airbag systems; follow OEM procedures for power-down to avoid accidental deployment.
• When working on data bus wiring, avoid creating short to power or ground. Disconnect the battery only after confirming appropriate vehicle-specific safety steps.
• Use appropriate personal protective equipment and ensure the vehicle is securely supported during any under-vehicle work.

What to document

• DTC: U0391 (and any accompanying codes)

• Symptom description and reproduction steps

• Vehicle make, model, year, and firmware/software levels

• Accessed modules and network map results

• Wiring/ground tests performed, with measurements

• Any OEM bulletins or software updates applied

• Repairs performed and parts replaced

• Validation steps and road-test results

• Any recommended follow-up or monitoring plan

• Wikipedia (OBD-II) - Diagnostic Trouble Codes: Provides general context for how DTCs are generated and the existence of network/communication codes like U-codes.

• Wikipedia (OBD-II) - Powertrain Codes: Context for where U-codes fit within the broader DTC framework and their role in powertrain diagnostics.

• General automotive diagnostic practice (industry knowledge): The outlined steps align with common field diagnostic techniques for CAN bus/network faults, including confirmation via scan tools, network mapping, wiring/ground checks, module isolation, and OEM software considerations.

• GitHub definitions (standard code information): Manufacturer-specific interpretation for U0391 and network/communication fault semantics; the exact module involved is vehicle-specific, and OEM documentation should be consulted for precise definitions.

• Real-world complaint patterns: No specific, accessible NHTSA data for U0391 ; the probability guidance above reflects ASE-level diagnostic experience rather than a published complaint distribution. If NHTSA/MA data for U0391 becomes available, it can be incorporated to refine cause probabilities.

Note on using sources

• It uses general ASE-level diagnostic practices for network/communication DTCs and emphasizes OEM-specific interpretation for U0391. If more vehicle-specific data becomes available, tailor the diagnostic approach to the exact module(s) implicated by the manufacturer for that vehicle.

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.

---