WOMS
U code network High severity

U0100 Lost Communication With ECM/PCM

A module on the vehicle CAN bus has lost communication with the Engine Control Module / Powertrain Control Module.

Definition

U0100 is a network communication code. The module reporting U0100 (could be the TCM, BCM, ABS module, etc.) is unable to talk to the PCM. This is usually a wiring/connector problem on the CAN bus, but can also be a failed PCM or a PCM power/ground issue.

Common causes

  • CAN bus wiring fault (broken wire, short, bad connector)
  • Failed PCM
  • PCM power or ground circuit fault
  • Failed CAN bus terminator resistor
  • Another module on the bus pulling the bus down

Common misdiagnoses

  • Replacing the PCM when the actual fault is a chafed CAN wire.

Troubleshooting steps

  1. 1. Verify PCM power and grounds

    Check fuses, power feeds, and chassis grounds to the PCM.

  2. 2. Measure CAN bus voltages

    CAN High should idle around 2.5V rising to 3.5V; CAN Low should idle around 2.5V dropping to 1.5V. Both stuck at the same voltage indicate a bus fault.

  3. 3. Disconnect modules one at a time

    If a faulty module is pulling the bus down, disconnecting it will restore communication.

  4. 4. Inspect bus connectors

    Look for green corrosion, water intrusion, or broken pins at the OBD2 port and major splice points.

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