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U codenetworkMedium severity

U0002High Speed CAN Communication Bus Performance

The high-speed CAN bus is still operating but with degraded or intermittent communication performance.

Definition

Modules on the high-speed CAN network monitor error rates, message timing, and retransmissions. U0002 sets when the bus is functional but performance is out of specification — excessive error frames, delayed messages, or intermittent dropouts rather than a complete failure. Symptoms are often intermittent: flickering warning lights, momentary gauge dropouts, sporadic loss-of-communication codes in other modules, and occasional harsh shifts or hesitation while data is delayed.

Common causes

  • Intermittent chafe or loose terminal causing momentary opens/shorts on the bus
  • Corroded or backed-out connector pins raising circuit resistance
  • A marginal module intermittently corrupting messages
  • Electrical noise coupling into the bus (damaged shielding, untwisted repair section, faulty alternator or ignition components)
  • Poor module grounds or low system voltage
  • Aftermarket devices tapped into the CAN circuits

Common misdiagnoses

  • Modules replaced for intermittent loss-of-communication codes they merely logged
  • PCM replacement for a connector or terminal-tension problem
  • Battery or alternator replaced when the fault is bus wiring
  • Instrument cluster replaced for intermittent gauge dropouts

Troubleshooting steps

  1. 1.Scan all modules and record the code history

    Intermittent U-codes scattered across several modules point to a shared wiring fault rather than one bad module.

  2. 2.Check bus resistance and voltages

    Verify about 60 ohms across DLC pins 6 and 14 key-off, and CAN High/Low idling near 2.6/2.4 V key-on.

  3. 3.Wiggle-test while monitoring live communication

    Watch scan-tool data or a lab scope while flexing harness sections and connectors; a dropout during movement localizes the defect.

  4. 4.Inspect terminal fit at bus connectors

    Check for spread female terminals, backed-out pins, and green corrosion at junction connectors, splice packs, and recently serviced modules.

  5. 5.View the bus on an oscilloscope if available

    Healthy CAN shows clean square dominant bits; ringing, rounded edges, or noise bursts indicate a termination, shield, or noise problem.

  6. 6.Disconnect aftermarket add-ons

    Remove remote starters, GPS trackers, or insurance dongles from the network and retest before condemning factory hardware.

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