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U0004High Speed CAN Communication Bus (+) Low

The CAN High (+) wire of the high-speed CAN bus is stuck at low voltage, typically from a short to ground.

Definition

CAN High should rest near 2.5 V and rise toward 3.5 V during dominant bits; when it is pulled low — usually by a short to ground in the harness or inside a module — differential signaling collapses and communication on that bus segment fails. U0004 sets in the detecting module, usually with a cascade of lost-communication codes. Symptoms commonly include multiple warning lights, inoperative gauges and systems, limp mode, or a crank/no-start.

Common causes

  • CAN High wire chafed through and shorted to chassis ground or a grounded shield
  • Water intrusion in a connector bridging CAN High to a ground pin
  • Crushed or pinched harness from a collision or previous repair
  • Internal short in a module's CAN transceiver dragging the line down
  • Incorrectly wired aftermarket accessory grounding the bus

Common misdiagnoses

  • PCM or BCM replaced when the short is in the harness
  • Multiple modules replaced chasing lost-communication codes
  • Battery or ignition switch replaced for a no-start caused by the dead bus
  • Instrument cluster replaced for dead gauges

Troubleshooting steps

  1. 1.Measure CAN High voltage at the DLC

    Key on, pin 6 to ground: a reading near 0 V instead of about 2.6 V confirms the line is pulled low.

  2. 2.Check for a short to ground

    Battery disconnected, measure resistance from DLC pin 6 to chassis ground; near 0 ohms confirms a hard short (a healthy bus reads high, through transceiver impedance).

  3. 3.Isolate by disconnecting modules one at a time

    Unplug each module on the bus while watching the resistance to ground; if it opens when one module is disconnected, that module's transceiver is shorted.

  4. 4.Split the bus at junction connectors

    If no module clears the short, open inline connectors to divide the harness and determine which physical segment carries the short.

  5. 5.Inspect and repair the damaged section

    Look for chafe points at brackets, grommets, and edges near the isolated segment; repair, insulate, and re-secure the loom away from the contact point.

  6. 6.Retest the network

    Confirm about 2.6 V idle on CAN High, roughly 60 ohms across pins 6 and 14 key-off, and full module communication on a rescan.

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