Diagnostic guide for OBD-II code U0336

Important Notes

• U0336 is a U-range DTC, which is a network/communications code. U-codes indicate issues on the vehicle's data bus or with module-to-module communication rather than a single physical sensor (these are "network" problems). This class of codes tends to be more about how ECUs talk to each other than about a single sensor or actuator.
• The exact meaning of U0336 can vary by OEM and vehicle architecture. In many makes, U0336 denotes a loss of communications with a specific module or a general loss of CAN bus communications. Always verify with OEM service information for the exact target module and any model-specific criteria.

Symptoms

• MIL may be illuminated or flash intermittently with multiple modules reporting "not communicating" or "lost communication" messages.
• In some cases, several vehicle modules (ABS, BCM, TCM, PCM, instrument cluster, gateway) show inconsistent data or show as absent in the scan tool.
• Intermittent drivability issues: random warning lights, inconsistent vehicle data on the dash, flaky feature operation that depends on module communication.
• Vehicle may not start or may run with limited functionality if critical modules are not communicating.
• After clearing codes, U0336 may reappear once the vehicle is driven or when a wake-up sequence occurs.

Probable Causes

• Wiring/connectors and CAN bus physical layer issues (loose, corroded, pin-melted connectors; damaged harness; broken shield/ground; abrasion on CAN lines): 35-50%
• Faulty module(s) or module software/firmware mismatch (one module failing to wake or dropping off; a mis-flashed ECU; a failed gateway/bridge module): 20-30%
• Power/ground reliability and power distribution problems (battery condition; bad ground strap; ignition/switched power issues; improper supply to one or more ECUs causing a module to drop off the bus): 15-25%
• CAN termination or network topology problems (improper or missing 120-ohm terminators; improper bus branching; multiple termination points): 5-15%
• Aftermarket devices or wiring causing electrical noise or bus disturbance (alarms, remote starts, infotainment mods): 5-10%
• OEM software/firmware updates required to restore proper bus communications (rare but possible in modern vehicles): 0-5%

What you should know before you diagnose (scope and priorities)

• U0336 is almost always a bus/communication issue rather than a single sensor fault. Look for other DTCs that share the same network (P, U, B, C codes) and note any "communications fault" patterns with specific modules.
• If more than one module shows "no communication" or you see "no data" in the scan tool, suspect CAN bus issues or a gateway/module wake-up problem. If only one module is affected, the fault may be isolated to that module or its direct wiring.
• OEMs may use different interpretation for U0336; always corroborate with the manufacturer service information for the vehicle you're working on.

Diagnostic Approach

1) Preparation and safety

• Ensure battery is healthy and fully charged. A weak battery can produce false bus faults.
• Use proper PPE and disconnect power only as required; avoid creating additional shorts when probing harnesses.
• Confirm the code's status: current vs history, and whether it reappears after a specific action (ignition on, road test, etc.).

2) Collect data and confirm scope

• Use an advanced scan tool capable of reading CAN bus status and module-specific communication status. Note any additional codes and freeze-frame data.
• Identify which modules are reporting communication issues (if the OEM tool provides a target module in the U0336 DTC, record that target).
• Check for any related U-codes (e.g., U0100, U0121, U0073) that indicate generic CAN issues or specific module communication faults.

3) Visual and mechanical inspection

• Inspect the CAN main trunk and module harnesses for obvious damage: chafed insulation, pin alignment, bent/flattened pins, corrosion, moisture intrusion, or aftermarket wiring.
• Inspect ECU/PCM grounds and power feeds. A poor ground or inconsistent power can cause modules to drop off the network.
• Check for aftermarket devices or recent repairs that could have disturbed the bus (alarm systems, remote starters, radios, amplifiers, etc.).

4) Electrical tests on the CAN network

• Confirm power and ground integrity to the suspected ECUs (supply voltage within manufacturer spec, stable ground continuity).
• Inspect CAN High (CAN_H) and CAN Low (CAN_L) wiring between the gateway or central node and the suspect modules. Look for opens, shorts to voltage, shorts to ground, or cross-talk.
• Measure the CAN bus with a scope if available:
  • Idle: CAN_H and CAN_L should show recessive state around typical bias levels (check the vehicle's service data for exact expected voltages; many CAN systems have a nominal bias around 2.5 V on each line in the recessive state).
  • When a message is transmitted, CAN_H goes high and CAN_L goes low (dominant state). Look for proper differential signaling and clean edge transitions.
  • Look for excessive bus errors, noise, or irregular waveform patterns that suggest wiring issues or terminal faults.
• Verify bus termination:
  • Ensure terminating resistors (~120 ohms) are present at both ends of each CAN segment as specified by OEM. Do not have multiple terminating networks or missing terminators, which can cause bus instability.
• Check for proper network topology:
  • Ensure no improper splices, incorrect T-bus wiring, or multi-ground scenarios that could cause signal integrity problems.

5) Module-specific checks

• If the OEM tool identifies a target module, individually check that module's wake-up/enable signals, clocking, and data consistency on the bus.
• Inspect the module's connectors for corrosion, bent pins, or damage; reseat connectors and apply dielectric grease if appropriate.
• If possible, perform a power-cycle test of the suspect module (disconnect the battery or relevant bus power while observing proper procedure) and re-connect to see if it regains communication.

6) Diagnosing with an end-to-end approach

• If multiple modules show loss of communication, focus on the gateway/central hub or the CAN backbone first, since a single fault there can cascade to many modules.
• If a single module is consistently not communicating, test just that module's interface and its direct wiring, then consider a module replacement or reflash if wiring checks pass.
• If all else checks out on the bus but U0336 remains, consider OEM software/firmware updates or reflash of involved ECUs. Some networks require modules to be updated to re-establish a reliable wake-up.

7) Tests to perform and order of operations

• Visual inspection of CAN wiring and connectors.
• Power/ground checks for all involved ECUs.
• CAN_H/CAN_L continuity tests and resistance checks.
• Oscilloscope trace of CAN_H and CAN_L for proper differential signaling and timing.
• Inspect termination resistors and bus topology.
• Module wake-up test and reseating or replacement as needed.
• Clear codes, drive vehicle, and recheck for recurrence.

8) Repair strategies (prioritize in this order)

• Repair or replace damaged wiring harnesses and connectors; reseat all relevant ECU connectors; fix grounds and power feeds to ECUs.
• Correct CAN bus issues: replace damaged sections, restore proper termination at the ends of the bus, remove erroneous splices, and ensure a clean bus topology.
• Replace or reflash a faulty module that is not waking or communicating correctly; ensure software level is compatible with other modules on the bus.
• If aftermarket devices are found to disturb the bus, relocate or remove them and re-test.
• After repairs, clear DTCs and perform a thorough road test; monitor for recurrence of U0336 and any related codes.

Post-Repair Verification

• Confirm absence of U0336 on multiple drive cycles and after a battery reset (if applicable).
• Validate that all modules are reporting correctly on the scan tool and that no new DTCs have appeared.
• If OEM software updates were applied, verify that the vehicle remains stable on the road and that no new network-related codes occur.

Safety Considerations

• A faulty CAN bus can affect critical systems; do not ignore symptoms such as ABS, airbag, transmission, or steering-related faults that appear alongside U0336.

• Disconnecting the battery or power to ECUs during testing can cause loss of frames or data-follow OEM procedures for power isolation.

• Work in a dry, clean environment; protect exposed wiring and connectors from moisture and impact.

• General DTC framework and the existence of U-codes (network/communication errors) are described in the OBD-II sections of Wikipedia (Diagnostic Trouble Codes; Powertrain Codes). These sources clarify that U-codes pertain to network communications rather than a single sensor fault.

• The guidance here aligns with the general diagnostic approach for network codes: verify the network, inspect wiring/connectors, check power/ground to modules, measure CAN bus signals, inspect termination/topology, and verify with OEM service information for vehicle-specific interpretation of U0336.

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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