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U0009High Speed CAN Communication Bus (-) Shorted to Bus (+)

The two wires of the high-speed CAN bus are shorted to each other, collapsing the differential signal.

Definition

CAN High and CAN Low must move in opposite directions to encode data; when the pair is shorted together the voltages lock at the same level and no dominant bits can be formed, taking down communication for the entire segment. U0009 is stored by the detecting module, and most or all modules on the bus drop offline with lost-communication codes. Symptoms are usually dramatic — many warning lights, dead gauges, inoperative transmission and stability systems, limp mode, or a no-start.

Common causes

  • Insulation chafed through between the two conductors of the twisted pair
  • Crushed or pinched harness pressing the pair together (collision, jack point, body clamp)
  • Water intrusion or corrosion bridging adjacent CAN pins in a connector
  • Melted loom from exhaust or electrical heat fusing the wires
  • Terminal misalignment or a bent pin after a module was serviced

Common misdiagnoses

  • Multiple control modules replaced for the cascade of lost-communication codes
  • PCM replacement when the fault is purely wiring
  • Battery and starter diagnosis for a no-start caused by the dead network
  • Terminating resistors replaced when readings actually indicate a wire-to-wire short

Troubleshooting steps

  1. 1.Measure resistance between the bus wires

    Key off, across DLC pins 6 and 14: a reading near 0 ohms instead of about 60 confirms the pair is shorted together.

  2. 2.Check bus voltages key-on

    CAN High and CAN Low sitting at identical voltage (both about 2.5 V with no separation) is the signature of a pair-to-pair short.

  3. 3.Disconnect modules one at a time

    Rule out an internal module short by watching the pin 6-14 resistance as each unit is unplugged.

  4. 4.Split the bus at junction connectors

    Divide the network into segments and measure each side to bracket which harness section contains the short.

  5. 5.Inspect and repair the pinch point

    Common locations are body grommets, seat tracks, brackets, and collision-repair areas; separate the conductors, insulate each, and restore the twist.

  6. 6.Verify recovery

    Confirm about 60 ohms across pins 6 and 14, normal 2.6/2.4 V idle voltages, and a full-system scan with every module reporting.

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