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All codes/U0100

U0100

Lost Communication With ECM/PCM 'A'

Critical — stop driving

This code means that one or more of the vehicle's computer modules has lost communication with the main engine and transmission computer (the ECM/PCM). It's like different parts of the car's 'nervous system' can't talk to its 'brain'.

Lost Communication With Engine Control Module/Powertrain Control Module 'A'

Common causes

  • Faulty ECM/PCM

    The engine's main computer itself may have failed, preventing it from communicating with other modules.

    high

    Typical part cost: $500-1500+ part

  • Wiring/Connector Issues (CAN Bus)

    Damaged, corroded, or loose wiring in the CAN (Controller Area Network) bus system can interrupt communication.

    medium

    Typical part cost: $0-300 repair

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First things to check

  1. 1Check all fuses related to the ECM/PCM and other modules on the CAN bus.
  2. 2Inspect the ECM/PCM connectors and wiring harness for damage, corrosion, or looseness.

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Tools you may need

Already have a compatible scanner? Use it. If not, a Bluetooth OBD2 adapter can help you confirm engine codes and check supported live sensor data before replacing parts.

Illustration of a Bluetooth OBD2 adapter, not a product photo

Basic checks

Veepeak OBDCheck BLE

A Bluetooth LE adapter for reading standard engine codes and supported live data.

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Illustration of a Bluetooth OBD2 adapter, not a product photo

Regular diagnostics

vgate iCar Pro BLE 4.0

A Bluetooth LE adapter with an automatic sleep feature for routine code and sensor checks.

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Illustration of a Bluetooth OBD2 adapter, not a product photo

Another option

OBDLink CX

A Bluetooth LE adapter to consider when comparing supported vehicles and diagnostic apps.

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Illustrations shown, not product photos. Check vehicle, phone and app compatibility before buying. These adapters do not guarantee access to ABS, airbags or manufacturer-specific systems. Clearing a code does not repair the fault and can reset emissions readiness monitors.

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Related codes

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Frequently asked questions

What is the CAN bus?

The CAN (Controller Area Network) bus is a communication network that allows different electronic control units (computers) in your car to share information and signals with each other efficiently.

Can a dead battery cause a U0100 code?

A severely weak or dead battery, or poor battery connections, can cause voltage fluctuations that lead to communication errors and trigger U-codes like U0100. Always check battery health first.

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