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DTC code page

U0100: Lost Communication With ECM/PCM “A”

Quick answer: A module on the network stopped receiving expected messages from the engine or powertrain control module.

Drivers also search this fault as lost communication with pcm, ECM communication lost, PCM offline code, U0100 PCM not communicating.

Severity: high Family: network Related paths: 14
Meaning

What U0100 usually means

U0100 means another module expected regular messages from the ECM or PCM and stopped seeing them. That does not automatically prove the engine computer died. It often means the PCM lost power or ground, the CAN network went unstable, or system voltage dropped low enough that the PCM could not stay online consistently.

Fast triage

Start here before chasing parts

  • Scan first: save freeze-frame and pending codes before clearing anything.
  • Confirm the complaint: compare the stored code with current drivability symptoms.
  • Use context: trims, live data, and related codes usually narrow the fault faster than guesswork.
  • Work simplest to hardest: leaks, connectors, maintenance items, and known patterns before expensive components.
Initial checks

What to check first

  • Treat PCM power and ground as first-tier checks because a dead feed can mimic a dead computer perfectly.
  • Ask whether the scan tool can talk to the PCM at all or only to other modules.
  • Check for battery-light, low-voltage, or PCM relay codes because they often explain why U0100 appeared.
Driving risk

Can you keep driving?

U0100 is serious because if the PCM cannot stay online, the vehicle may not start or may lose major powertrain functions. Diagnose it as a priority, especially if the problem is active now.

High urgency: If symptoms are active, reduce driving and diagnose quickly before secondary damage builds.
Likely causes

Common causes behind this code

  • PCM power relay, fuse, or ignition-feed fault knocking the PCM offline
  • Poor PCM ground or voltage drop in the main power path
  • CAN network wiring fault preventing other modules from hearing PCM traffic
  • Low battery or unstable charging voltage causing module resets
  • Actual PCM failure after water intrusion or internal damage

Cause phrases often tied to this code: PCM power relay, lost PCM ground, CAN network issue, low battery voltage, fuse box corrosion.

Diagnostic order

Suggested workflow

  1. Scan every module and confirm which modules report U0100 versus which modules are completely offline.
  2. Verify PCM battery feed, ignition feed, relay output, and grounds under load.
  3. Inspect the CAN lines at the PCM connector and main network junctions for shorts, opens, or corrosion.
  4. Check battery and charging performance so a wider voltage problem is not missed.
  5. Only consider PCM replacement after powers, grounds, and network integrity are proven.
Avoid guesswork

Common mistakes

  • Replacing the PCM because U0100 sounds dramatic without testing the relay and feed circuits first.
  • Ignoring low-voltage evidence that would explain why the PCM dropped offline.
  • Focusing only on one module instead of mapping the whole network communication picture.
Repair path

Practical fix guidance

  • Restore stable PCM power, ground, and network integrity before replacing control modules.
  • Repair fuse-box corrosion, loose connectors, or relay-feed issues that interrupt PCM wake-up.
  • After repair, verify other modules can communicate with the PCM and that the original no-start or reduced-power symptom is gone.
Vehicle context

Affected brands in this MVP

Brand hubs help broaden internal linking now and can evolve into make-specific diagnostic notes later.

Aliases and common searches

English phrases tied to U0100

Useful when the driver knows the wording but not the exact DTC yet.

  • lost communication with pcm
  • ECM communication lost
  • PCM offline code
  • U0100 PCM not communicating
Related search intent

Queries this page can answer naturally

  • U0100 code meaning
  • what does U0100 mean
  • lost communication with ECM PCM A
  • PCM communication failure symptoms
FAQ

Quick questions about U0100

Does U0100 mean I need a new PCM?

Not by itself. Power, ground, relay, and CAN issues are common reasons the PCM disappears from the network.

Can U0100 cause a crank-no-start?

Yes. If the PCM is offline, the engine may crank normally but never receive proper fuel, spark, or authorization commands.

Why does U0100 often appear with low-voltage codes?

Because unstable system voltage can reboot or silence the PCM and make other modules report lost communication.