Comprehensive diagnostic guide for OBD-II code U0135

Disclaimer on scope and sources

• U0135 is a U-code (network/communications code) in the OBD-II framework. U-codes indicate issues on the vehicle's data network (CAN/HS-CAN, etc.) rather than a single physical sensor or actuator. The exact module involved is typically not defined by the code itself and depends on the vehicle's network topology.
• Standard code information (including U-codes) is summarized in the general OBD-II references; the exact module affected is often determined by vehicle make/model and the network architecture.
• No specific NHTSA complaint data for U0135 is provided . Where possible, probabilities are based on general field experience and typical network-code behavior, with caveats that results are highly vehicle-specific.

1) What U0135 means (definition and scope)

• Type: U-code (network/communications code) in OBD-II.
• General meaning: Lost or failed communication on the vehicle's data network between modules. The diagnostic trouble code indicates that one or more modules did not respond or could not communicate within the expected timeframe on the network (CAN bus and/or other internal networks).
• Important caveat: The code does not specify which module is at fault; a single faulty module, a gateway/needs-to-be-respected controller, a damaged bus segment, or a power/ground issue can cause multiple modules to fail to communicate.

2) Common symptoms and real-world complaints

• MIL (Check Engine Light) or DTCs present for U0135 or multiple U-codes indicating loss of network communications.
• Several systems report "no data" or "no communication" from multiple control modules in scan tools.
• Modules that normally provide data (e.g., ABS/Traction, Instrument Cluster, Transmission, ECU/PCM, HVAC, Airbags) may illuminate their own fault lights or display degraded behavior due to lost communications.
• Vehicle may start and run erratically or limp mode is triggered if network communications are critical to control logic.
• Cruising/diagnostic symptoms may include intermittent or persistent loss of data on the scan tool, especially when the vehicle is reset or woken after a power interruption.

3) Primary causes (categories) and probable likelihoods

Note: Since U0135 and similar network codes depend heavily on vehicle architecture, the exact cause may vary. Probabilities below are general field estimates and should be refined with vehicle-specific data.

• Wiring, connectors, and harness issues on the CAN/communication network

  • Likelihood: high (roughly 25-50%)
  • Examples: pin-to-pin harness damage, poor connectors, bent or pinched wires, corrosion, water ingress, or frayed insulation on CAN High/Low or gateway lines.
    Why it fits: damaged or dirty connections often produce intermittent or total loss of module communications.

• CENTRAL gateway/module or ECUs/transceivers failing on the bus

  • Likelihood: moderate to high (roughly 20-40%)
  • Examples: a faulty CAN transceiver inside an ECU, a failing gateway/bridge controller, or a module that intermittently shuts down the bus.
    Why it fits: one bad node can cause multiple modules to "go silent" on the network.

• CAN bus termination or physical layer issues

  • Likelihood: low to moderate (roughly 5-15%)
  • Examples: missing, damaged, or incorrect 120-ohm termination on the bus; shield grounding issues; bus topology problems.
    Why it fits: improper termination or a degraded bus can prevent reliable communication.

• Power/ground supply problems to networked modules

  • Likelihood: moderate (roughly 10-25%)
  • Examples: battery voltage drop, poor ground connections, voltage spikes/dips on ignition-on/off transitions, fusible links or supply rails failing to reach modules.
    Why it fits: many ECUs rely on stable supply rails; low or unstable power can cause modules to reset or fail to communicate.

• Software/firmware or calibration issues

  • Likelihood: optional but possible (roughly 5-15%)
  • Examples: outdated or corrupted firmware on one or more modules, need for software updates, or calibration mismatches after repairs.
    Why it fits: software problems can cause a module to appear "offline" or to mis-handle bus messages.

• Other indirect causes

  • Examples: aftermarket devices, improper wiring modifications, or environmental factors (extreme heat, moisture) impacting the network.

4) Diagnostic approach: step-by-step procedure

Plan: start with broad checks (power/ground and physical network), then narrow to the bus and individual modules. Use a good OBD-II scan tool capable of reading live CAN data and, if possible, OEM/advanced data logging.

Prepare and verify

• Confirm DTCs: Read all current DTCs and any pending codes; note any other U-codes or P/B/C codes present. Freeze frame data can give context (engine RPM, vehicle speed, battery voltage, etc.).
• Vehicle safety: ensure ignition off during inspection before disconnecting any connectors unless required, and follow all safety procedures (battery disconnect, airbags precautions if applicable).

Visual and power/ground checks

• Inspect battery, alternator, and main fuses; verify battery voltage is within spec with ignition on and engine running.
• Inspect power and grounds for the modules on the suspected network (common ground points, chassis grounds, battery negative cable).
• Inspect for aftermarket wiring that could load or disrupt the CAN bus.

Inspect the CAN data network (physical layer)

• Visual: inspect CAN High (CAN-H) and CAN Low (CAN-L) wiring for damage, chafing, or pin damage at connectors, especially near modules, underhood, dash, and behind the instrument cluster.
• Connector integrity: reseat or replace damaged/dirty connectors; check for corrosion.
• Termination: verify there is proper bus termination (a 120-ohm resistor at each end of the bus). Missing or damaged terminators can cause communication faults.
• Shielding and routing: ensure shielded lines are properly grounded and not cut or pinched.

Use a data-logging and bus-scope approach (live data)

• With a capable scan tool or oscilloscope, monitor CAN-H and CAN-L activity while the ignition is on and during a drive cycle.
• Look for:
  • Consistent activity on both CAN-H and CAN-L lines with correct differential signaling when the bus is active.
  • Recessive idle state: CAN-H and CAN-L near mid-supply (~2.5 V each) with minimal differential.
  • Dominant state: CAN-H rises while CAN-L falls during message transmission.
  • Any lines that show abnormal voltages, stuck states, or lack of activity on one line.
• Identify modules that fail to appear on the network or drop offline during testing.

Identify suspect modules or the gateway

• If one module consistently does not communicate while others do, suspect that module or its transceiver.
• If multiple modules intermittently go offline together or all fail after power-up, suspect the gateway/central controller or a bus topology problem.
• Where feasible, swap or bench-test a known-good module (or use a known-good gateway) to determine if the issue is module-specific.
• Be mindful that some vehicles rely on a gateway module to translate or route messages; a fault in the gateway can produce widespread "no comm" indications.

Power/ground and software considerations

• If power/ground checks look good but network issues persist, consider a software/firmware update on one or more modules (manufacturer-specific).
• If you recently performed repairs or installed aftermarket devices, double-check that the modifications did not intrude on the CAN network (e.g., unauthorized splices, added devices on the bus, or altered grounds).

Verification and confirmation

• Clear codes after repair (if appropriate) and perform a road test to observe that the modules stabilize and re-establish communications.
• Re-scan to ensure no new U-codes appear and that prior U0135 clears if the issue is resolved.
• Confirm that other related systems function normally (ABS, instrument cluster, transmission data, etc.) during test drive.

Documentation of findings

• Record all observations: voltages, connector conditions, wiring faults found, modules tested, any swapped components, software update status, and test-drive results.
• Capture freeze-frame data and any live data logs showing bus activity and module presence.

5) Practical repair actions (typical remedies)

• Repair wiring harnesses or connectors showing damage; reseat and clean connectors; replace corroded pins or connectors.
• Replace or re-seat a faulty module or gateway on the bus if testing isolates a specific node.
• Correct CAN termination issues (install or repair 120-ohm terminators as required by the bus topology).
• Repair or improve power/ground paths to the affected modules (secure grounds, fix battery/alternator issues, address voltage drop).
• Apply manufacturer-recommended software/firmware updates to modules as needed.
• After repairs, recheck the entire network with live data and clear codes; test drive to confirm stable network operation.

6) Safety considerations and professional practice

• Always disconnect the battery when performing invasive electrical diagnostics or wiring repairs, and follow proper ESD and safety procedures.
• In hybrid/EV or high-voltage vehicles, follow specific safety guidelines to avoid electric shock or damage to high-voltage systems.
• Avoid creating or amplifying CAN faults with improper bench tests or unshielded wires; use appropriate measurement tools (scopes, breakout boxes) and minimize test leads near vehicle power lines.
• When in doubt, consult vehicle-specific service information for network topology, module locations, and OEM diagnostic procedures.

7) What to communicate to the customer

• U0135 indicates a network communication issue on the vehicle's data bus. The root cause could be wiring, a gateway or module fault, a bus termination problem, or a power/ground issue.
• Diagnosis requires a methodical inspection of the CAN bus and modules, and repairs may involve wiring repairs, module service or replacement, or software updates.
• After repairs, codes should be cleared and a road test performed to verify restoration of normal communications.

8) Probable causes and their rough likelihoods (customer-facing summary)

• Wiring/connectors on the CAN bus (high likelihood): 25-50%
• Gateway/module or CAN transceiver fault on the network (high likelihood): 20-40%
• CAN termination or physical layer issues (low to moderate): 5-15%
• Power/ground supply issues to networked modules (moderate likelihood): 10-25%
• Software/firmware issues (lower likelihood, vehicle- and OEM-specific): 5-15%
  Note: These ranges are approximate, vehicle-dependent, and reflect typical field observations for network-related DTCs like U0135. If NHTSA complaint data were available for U0135, we would factor that into the probabilities; in its absence, experience-informed estimates are provided.

9) Quick reference checklist

• Read all current and pending DTCs; note freeze-frame data.
• Inspect power, grounds, fuses, and battery condition.
• Visually inspect CAN bus wiring, connectors, and terminations; repair as needed.
• Check for OEM software/firmware updates on involved modules.
• Use a scope/diagnostic tool to observe CAN-H and CAN-L activity; identify any modules that fail to appear on the network.
• Isolate the fault by testing known-good modules or substituting components where safe and feasible.
• Clear codes, test drive, and re-scan to verify stable communications.

References used

• Wikipedia - OBD-II: Diagnostic Trouble Codes (overview of DTC categories and the existence of network (U-code) codes in OBD-II). This supports the understanding that U-codes are network-related.
• Wikipedia - OBD-II: Powertrain Codes (context for OBD-II code categories and how U-codes fit into network diagnostics). This helps clarify the general nature of network codes within OBD-II.
• General automotive diagnostic practice (industry-standard concepts referenced in the guide) reflects network-domain troubleshooting steps such as verifying power/ground, inspecting CAN bus, and using live data logging.

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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