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U0008High Speed CAN Communication Bus (-) High

The CAN Low (-) wire of the high-speed CAN bus is stuck at high voltage, typically from a short to battery power.

Definition

When CAN Low is pulled up near battery voltage instead of idling around 2.5 V, the differential relationship between the two bus wires is destroyed and modules on the segment stop communicating. The detecting module sets U0008, typically with widespread lost-communication codes, and a sustained 12 V on the line can damage transceivers in connected modules. Symptoms include multiple warning lights, dead gauges, limp mode, or a no-start until the short is repaired.

Common causes

  • CAN Low wire chafed against a battery-voltage circuit in the harness
  • Melted or contaminated connector bridging CAN Low to a power pin
  • Failed module transceiver pulling the line to its supply rail
  • Collision or heat damage merging bus and power wiring
  • Incorrect aftermarket wiring feeding 12 V onto the bus

Common misdiagnoses

  • Modules replaced before the harness short to power is located
  • PCM replacement driven by the number of stored network codes
  • Charging system repairs for symptoms caused by the failed bus
  • DLC or gateway replacement without voltage testing

Troubleshooting steps

  1. 1.Measure CAN Low voltage at the DLC

    Key on, pin 14 to ground: a steady reading near 12 V instead of about 2.4 V confirms a short to power.

  2. 2.Test key-off to classify the feed

    If 12 V remains with the key off, the bus is contacting an unswitched battery circuit, which narrows the candidate wiring.

  3. 3.Disconnect modules one at a time

    Monitor pin 14 voltage as each module is unplugged; a return to normal identifies a module with a failed transceiver.

  4. 4.Split the bus to isolate the harness section

    Open junction connectors to determine which physical segment carries the wire-to-wire short.

  5. 5.Repair the damaged wiring

    Find the melt-through or chafe where the bus runs with power feeds, repair both circuits, and reroute or protect the loom.

  6. 6.Confirm no collateral module damage

    Rescan after repair; any module still silent despite good power, grounds, and bus wiring likely has a burned transceiver.

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