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Vehicle network communicationSafety screen firstReviewed July 15, 2026
U0100

Lost communication with ECM/PCM A

A controller reported that expected communication with the engine or powertrain control module was lost for a qualifying period.

01

Conditions that can produce U0100

These are diagnostic branches, not a ranked parts list. Prevalence and testing order depend on the exact vehicle and failure scene.

  • Low system voltage or reset event
  • ECM/PCM power, ignition feed, or ground loss
  • CAN or other network open, short, resistance, or interference
  • Gateway, connector, water, or harness problem
  • Module programming, internal fault, or network overload
02

What to inspect next

  1. Stop for stalling, no-start, unsafe control loss, heat, or electrical odor
  2. Identify which modules reported the code and when
  3. Check voltage and ECM/PCM feeds before blaming the network
  4. Use exact topology, resistance, voltage, waveform, and isolation procedures
03

Evidence that strengthens the answer

Multi-module code timestamps and statusBattery and controller supply during eventNetwork topology and physical inspectionBus waveform/resistance and module-isolation result
04

Verify the repair

Preserve the pre-repair record, document exactly what changed, then repeat the relevant operating condition safely. Confirm no current or pending fault returns, review permanent-code and readiness state where applicable, and state which tests or modules remained unavailable.

RELATED METHOD GUIDE

How to diagnose an intermittent vehicle problem

Use event conditions, repeated scans, voltage and network evidence, controlled logging, and non-destructive testing to catch faults that disappear.

Read the full guide ↗
DEFINITION LIMIT

This page explains a commonly standardized code at a general level. Confirm the exact definition, module, status, test conditions, thresholds, wiring, and service process for the vehicle application using current official information.