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U0003High Speed CAN Communication Bus (+) Open

An open circuit has been detected in the CAN High (+) wire of the high-speed CAN bus.

Definition

High-speed CAN transmits data differentially over a twisted pair, and an open (break) in the CAN High conductor prevents proper dominant/recessive signaling for every module beyond the break. The detecting module sets U0003, and modules on the far side of the break drop offline, typically logging lost-communication codes. Depending on where the break is, symptoms range from a few inoperative systems to multiple warning lights, dead gauges, limp mode, or a no-start.

Common causes

  • Broken CAN High wire from chafing, rodent damage, or a pinched harness
  • Backed-out, bent, or corroded terminal at a module or inline connector
  • Harness damage from collision or previous repair work
  • Poorly made splice from an accessory installation that has failed
  • Open circuit inside a module's connector or internal board (less common)

Common misdiagnoses

  • Control modules replaced when a harness break is the fault
  • PCM replacement based on lost-communication codes in other modules
  • Instrument cluster replaced for dead gauges caused by the open
  • Battery/charging system service for a pure wiring failure

Troubleshooting steps

  1. 1.Map the offline modules with a network scan

    The break lies in the bus segment between the last responding module and the first missing one on the network topology diagram.

  2. 2.Measure termination resistance at the DLC

    Key off, read across pins 6 and 14: about 120 ohms instead of 60 suggests one terminating resistor has been cut off by an open in the bus.

  3. 3.Continuity-test CAN High through the suspect segment

    With the battery disconnected, check DLC pin 6 to the CAN High terminal at each unreachable module; more than about 1 ohm per run indicates a problem.

  4. 4.Inspect connectors along the segment

    Open junction connectors and splice packs in the affected area and look for backed-out terminals, corrosion, and broken locks.

  5. 5.Repair with a proper splice

    Solder and seal or use OE-style crimp splices, preserving the twist of the pair (roughly one twist per inch) to maintain noise immunity.

  6. 6.Clear codes and verify all modules communicate

    Rescan the network after repair and confirm no U-codes reset during a road test.

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