Comprehensive diagnostic guide for OBD-II code U0136

Overview

• What U0136 is in general: U-codes are "network/communications" fault codes in the OBD-II framework. They indicate the vehicle's modules are not communicating properly over the vehicle data network (often CAN). The exact module(s) involved are OEM-specific and depend on the vehicle's network topology (ECM/PCM, TCM, ABS, BCM, IPC, etc.). This is consistent with the OBD-II/Powertrain code framework described in the Wikipedia OBD-II references.

  • Source context: OBD-II diagnostic trouble codes and the concept of Powertrain Codes explain that DTCs include network/communications codes (U-codes) and that powertrain codes cover the engine, transmission, and related controllers. See: OBD-II - Diagnostic Trouble Codes; OBD-II - Powertrain Codes.

• How U0136 is typically used: It signals a loss of communications on the vehicle's data bus between modules. Because U0136 is a network code, the exact module(s) failing to respond can vary by vehicle make/model. OEMs add specific descriptions for which module is not communicating, so you may need OEM tooling or a server/TSB reference to identify the exact module on this vehicle.

• What you should do: Treat U0136 as a network bus fault rather than a single-module "defect." The root cause is typically wiring/connector issues, a failing module, power/ground instability, or a bus termination/power stability issue. You'll verify by inspecting the CAN/LIN (or other bus) network and its power/ground references, and by validating that all modules on the bus are powered and grounded correctly.

Symptoms

• MIL illuminated or pending DTCs with U0136 (and possibly other U-codes)
• Multiple systems appear to be unavailable or "offline" in the vehicle (e.g., ABS, BCM, IPC, ECM/PCM, TCM, or other modules stop communicating)
• Inconsistent or no readings in scan tool data from modules that should be on the same network
• Intermittent or complete loss of certain vehicle functions (e.g., traction control, ABS signals, dashboard communications) while driving
• Aftermarket devices tied into the data network may cause or contribute to bus contention or ground/power disturbances

Note: give general descriptions of OBD-II codes and powertrain codes but do not list a vehicle-specific symptom table for U0136. Use the generic symptoms above and observe what the specific OEM vehicle does on your test subject.

When to consider NHTSA complaints data (availability and use)

• If you have access to NHTSA complaints or vehicle-specific failure patterns, use those to adjust the likelihoods I provide below.

Probable Causes

Note: These are approximate, vehicle- and network-specific. Use them as a guide rather than absolutes.

• Faulty or damaged CAN bus wiring/terminals or damaged connectors in the network harness (including pin damage, corrosion, insulation wear, or water ingress). 40-55%
• One or more module faults on the network (ECM/PCM, ABS/ESP, BCM, TCM, IPC, etc.) that are failing to respond or are intermittently failing to talk on the bus. 15-25%
• Power supply or grounding issues affecting multiple modules (low battery voltage, poor grounds, or voltage drop under load). 10-20%
• Ground and/or power distribution issues (shared ground points, chassis grounding problems) leading to intermittent bus activity. 5-15%
• Aftermarket devices or non-OEM wiring components that interfere with the CAN bus (adapters, alarms, remote start modules, etc.). 5-15%
• Firmware/software mismatch or a faulty ECU's CAN transceiver (less common but plausible if modules are old/updated or if a software update occurred). 5-10%

Important Notes

• The percentages above are approximate and meant to guide diagnostic focus based on typical field experiences when no vehicle-specific data is available.
• Always verify with vehicle-specific factory/OEM data when available, because some vehicles use multiple buses (CAN, LIN, FlexRay, etc.) and have different network topologies.

Step-by-Step Diagnosis

1) Confirm the DTC and collect initial data

• Use a capable OBD-II scan tool that can read live CAN data and show networks. Confirm U0136 is present and note any additional DTCs (other U-codes or P/B/C codes).
• Record freeze-frame data if available and any current vehicle operating conditions (engine on/off, speed, gear, battery voltage, etc.).
• Check for OEM-specific DTC descriptions if the tool provides them; be prepared to cross-check with OEM service literature or GitHub-style code definitions that indicate a "lost communications" type code.

2) Safety and initial checks

• Check battery condition and charging system. A weak battery or fluctuating voltage can cause modules to drop off the network.
• Inspect for aftermarket devices connected to the data network that could cause bus contention or interference.
• Inspect for obvious wiring issues: loose connectors, damaged harness segments, signs of heat, water intrusion, or rodent damage near modules.

3) Visual inspection of the CAN network topology

• Identify likely network backbone(s) and end modules. Common modules on a CAN bus include ECM/PCM, ABS/ESP, BCM, TCM, IPC (instrument cluster), and possibly other control modules.
• Look for obvious wiring problems around the main harnesses and at each module connector. Pay attention to grounds and power feeds to the modules.

4) Power, grounds, and module health checks

• Verify battery voltage is stable (typical healthy systems near 12.6+ volts when engine is off, 13.5-14.8 volts with engine running). If voltage dips under load, address battery/alternator or parasitic draws.
• Check ground integrity from each module to the chassis and engine block. Poor grounds can cause intermittent bus faults.
• With the engine on, observe each module's supply voltage (if your scan tool or a DMM can show module power feed status). Look for modules that drop out of the network when loads increase or when certain functions are exercised.

5) CAN bus health testing (non-destructive)

• Measure CAN_H and CAN_L at several points along the bus (where accessible). Look for:
  • Proper continuity between the bus segments (no open circuits)
  • No shorts from CAN_H or CAN_L to battery positive or to ground
  • Presence of termination resistors (120 ohms) at the network ends; an open or missing termination can cause communication instability
• If you have an oscilloscope or a high-end scan tool, inspect CAN_H and CAN_L activity for proper differential signaling and bus idle state when the vehicle is in a known idle state.

6) Module-by-module and bus-by-bus verification

• Using the scan tool, try to ping or request data from modules one by one (ECM, ABS, BCM, IPC, TCM, etc.). Note which module responds and which do not.
• If one module consistently does not respond while others do, focus on that module's power/ground and its physical connectors first, then its CAN transceiver.

7) Look for corroborating DTCs and history

• If other DTCs are present (P-codes for communications, U-codes for other modules, or C-codes), use them to narrow the likely network section that is failing.
• Check for pattern: if many modules intermittently talk and then drop, suspect bus integrity; if only a single module is dead, suspect that module's transceiver or its wiring to the bus.

8) Targeted repair steps (based on findings)

• Wiring/connectors: repair or replace damaged harness sections; reseat connectors; apply dielectric compound or heat-sink compounds where applicable; repair or replace damaged grounds.
• Module power/ground: repair any damaged power supply lines or grounds to affected modules; ensure proper voltage levels and no voltage drop under load.
• Module replacement/flash: if a specific module's CAN transceiver is suspected (due to repeated failure to communicate, repeated DTCs from that module, or a known OEM service bulletin), consider replacement or reflash/updates per OEM guidelines. Note: reprogramming may require dealer tools or vendor-specific software.
• Termination and network integrity: ensure correct 120-ohm terminations at both ends of the CAN network. If a termination resistor is missing or out of spec, add or replace it per vehicle topology.
• Aftermarket interference: remove any aftermarket devices interfacing with the network and re-check.

9) Post-repair verification

• Clear DTCs (after repair is complete) and perform an extended road test, simulating typical operation scenarios (start/stop, speed changes, idle with accessories on, etc.).
• Re-scan to confirm U0136 does not reoccur and that other modules remain healthy.
• Confirm that the hospital-scale symptom set (modules talking, appropriate data on PIDs, and proper vehicle behavior) has returned to normal.

Manufacturer Notes

• OEM network layouts vary widely. Some vehicles share multiple CAN buses (high-speed CAN for powertrain, low-speed CAN for body/electrical modules). In such cases, U0136 could reflect a problem on any bus or with a bridge between buses.
• Some makes use LIN or other local networks in addition to CAN; a fault could be on a secondary network and still present as a U-code depending on the diagnostic tool's interpretation.
• Always consult the vehicle's service information and TSBs for model-specific network topology, recommended test procedures, and any known wiring harness concerns or module reliability issues.

Tools and tests you'll want

• A capable OBD-II scanner with CAN bus support and the ability to view live data and DTCs across modules.
• Multimeter or oscilloscope for CAN bus signal testing (differential CAN_H/CAN_L activity, voltage levels).
• Basic hand tools for wiring harness inspection (screwdrivers, pry tools, dielectric grease, electrical connectors, replacement pins if needed).
• A dedicated 120-ohm CAN termination resistor (for diagnostic topping or test provisioning, if needed) and knowledge of where end-term resistors should be.
• OEM service information or a reliable code database.

Safety Considerations

• Always follow standard safety procedures when servicing automotive electrical systems.
• If powering off the vehicle, ensure proper lockout/tagout for battery work; avoid inadvertent airbag deployment when disconnecting components near SRS systems.
• Avoid shorting CAN lines to power or ground; use proper isolation and avoid live electrical testing when possible.
• When disconnecting any module connectors, label them and document pinouts to ensure correct reassembly.

Documentation

• Record the exact DTCs (U0136 and any others), vehicle VIN, OBD-II connector state, battery voltage, and charging voltage.
• Note all wiring and connector observations, including photo documentation of any obvious damage or corrosion.
• Document the exact repair actions taken (which modules were checked/replaced, wiring repairs performed, firmware updates completed, etc.).
• After repair, record the post-repair test results and a verification drive.

Documentation

• A loss of communications on the vehicle data network is typically caused by wiring/connectors, a faulty module, or a power/ground issue. In many cases, the culprit is a flaky connector or damaged wiring; in others, a module or its transceiver needs repair or replacement, sometimes alongside firmware updates.

• Reassure them that the diagnostic process targets the network as a whole and that the fix will focus on restoring proper module communication, followed by a verification drive.

• General OBD-II code framework and the concept of U-codes (network/communications codes) are presented in the OBD-II Diagnostic Trouble Codes and Powertrain Codes sections of on OBD-II. This supports the understanding that U0136 is a network communication code and that the exact module involved is OEM-specific. See: Wikipedia - OBD-II - Diagnostic Trouble Codes; Wikipedia - OBD-II - Powertrain Codes.

• The approach to treating U-codes as network/communications issues and focusing on bus health, power/ground integrity, and module communication is aligned with the broader understanding of OBD-II networks described in the same sources.

• For standard code interpretation and naming conventions, GitHub-style definitions of OBD-II codes describe U-codes as network/communication problems; the exact OEM module reference is vehicle-specific.

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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